So... building a fighter to be good at combat, means that they can only be good at combat. And that is because of the players, not because of the game design.
Optimizing for combat by having a high Constitution will make you bad at other puillars. I would call that building for combat.
So... you are making my point for me?
I mean, you are literally sitting here telling me that are level 15 the party is facing SKELETONS, CR 1/4 skeletons. Level 18 has the very scary CR 5 Vampire spawn, surely a terrifying fight.
Exactly those are not relevant enemies that you can use those spells on.
You said your spells are still OP. Using Dominate Monster to take control of a CR 1/4 skeleton is irrelevant and the Vampire or Dragon you want to use it on to be an "encounter ender" has legendary resistance.
When you get above 15th level just about every encounter that matters has enemies with Legendaries, significant condition immunities, magic resistance with high saves. Most of the time it is more than one of these.
I mean seriously, that level 17 list doesn't have a single monster with a CR higher than 10. So... why does the legendary resistance matter? You keep acting like the party is only ever going to face foes with high saves and legendary resistance, then offer a list that showcases that the majority of enemies... won't have those things.
The majority of enemies that matter do have those things.
Wut? How is "Not being able to move, all attacks against you have advantage, all of your attacks have disadvantage, you have disadvantage on dex saves"... not very bad for a lot of enemies?
It is bad for some, but not a lot and if it is really bad and they fail, and they have a legendary they will use it.
I really do not think you play a whole lot at high level. Here are some of the reasons it is not as debilitating as you think:
1. Teleports/plane shifts and off turn movement are relatively common at this level. So he simply leaves the web with movement before his turn or by teleporting on his turn right after it affects him but before the party can benefit.
2. It does not take effect until the start of the enemies turn: For example, right after the Wizard's turn, the Blue Dragon can use a Legendary action to make a Wing Buffet and fly up to his speed to leave the web and this would happen BEFORE his turn, BEFORE he makes the save and BEFORE he is restrained. Let's say he only has 1 legendary action left and can't buffet. On the next turn Rogue shoots an arrow, that is not with advantage because he is not restrained yet. Then after the Rogues turn the Dragon uses a Legendary action to use a tail attack and that is not with disadvantage because he is not restrained yet. The fighter (Eldritch Knight) decides not to close because he doesn't want to get in a web, he uses ICE STORM from his wand of Winter, the saving throw is not with disadvantage because he is not restrained yet. It finally gets to the Dragon's turn and he needs a 13 to save. If he fails he is finally restrained if he decides not to use a legendary action.
3. Restrained is not as debilitating as at earlier levels: Advantage has less effectiveness when you hit most of the time and PCs will be hitting most of the time. Disadvantage is not as effective because enemies have non attack options. Looking at the dragon above his has a 1 in 3 chance of getting his breath back and not even needing to attack and he knows this before he decides to use a legendary to pass the save.
So... 40% chance to restrain the dragon, preventing all melee attacks against the party.
No. Not at all.
1. 60% chance to make him choose whether or not he wants to use a legendary to pass.
2. He can attack out to 15 feet while restrained and he can attack using legendary action after you cast web but before his turn without disadvantage.
3. He has Lightning Breath, if you cast this the first turn of combat, this is going to be what he does anyway (likely ending your concentration). If it is not the first round he has a 1 in 3 chance of getting back his lightning breath, but he knows this BEFORE he has to decide on using a legendary.
In exchange, the fighter has triggered their bonus action which is much more limited.
Are you just making things up? It is less limited.
I used PB+5 in the save DC (otherwise the Dragon's save is easier). A RK can use Giant's might 5 times a day. A Wizard gets 3 2nd level spells a day.
gave up two of their three attacks
Yes probably they gave up PART of their action and provided a more debilitating condition far more reliably and with less chance of being broken. They still have a peice of their action left (unlike the Wizard).
, is locked in melee with the dragon, and can only attack if they have a hand free.
This is simply false. They do not need a hand free to attack.
They need a hand free to grapple and it is a strawman considering the Wizard needs concentration and a free hand to cast web and the concentration being available in particular is going to be more situational manage than the free hand.
This means that they are not using a shield
No it doesn't. Where do you get this from. You can grapple with one hand free. You can have a shield or whatever you want in the other.
so, without assuming magical weapons they have an AC of 18. Adult Blue Dragons have +12 to hit, and even if they still have disadvantage they only need to roll a 6, on three attacks.
This math is the same if they are in a web.
Add in all the ways the dragon has to knock the fighter prone or blind them
He can try to knock him prone off turn, but the Dragon is still grappled and prone and the RK can just stand up on his turn.
If the Dragon tries to shove the fighter away to break the grapple it is likely to fail and that takes all 3 attacks to do because grapple or shove are an entire action for monsters.
I am not tracking how he can blind him. I suppose he could use his action to improvise a skill check of some kind.
And in exchange, he can't move, but he also is harder to target with ranged attacks, meaning everyone has to get into melee with him. Which means he can do the same things to them that he can do to the fighter.
Not being able to move is huge for a creature that can move 80 feet, burrow 30 feet and move 40 feet off turn. Allies do have disadvantage on attacks from beyond 5 feet, but as noted above that is not very debilitating at this level.
Also don't discount what the fighter can do moving the Dragon around the battlefield.
Also, sidenote, +7 vs +10 with advantage is rough for them, but I'm not sure it is 90% chance to fail.
It is over 90% to fail at least 2 out of 3 (fighter makes at least 2 out of 3). I am assuming the fighter uses all three attacks for that if he needs to.
The fighter gets 3 attacks, to not be prone and grappled the dragon needs to succeed on the first two checks (third is irrelevant), on the first and third check or on the second and third check. That is about 9% chance of those three combined. Note this includes ties going to the dragon (as per the contest rules).
And, if you just give the dragon athletics proficiency that becomes +12 vs +10 with advantage, and that is much more even.
Sure if it is not an RAW dragon. There are very few monsters that have athletics proficiency.
If casters are always, by design, more powerful. What is the point of the martial characters?
Fun to play!
So what? Being weaker at one specific level doesn't mean that they aren't stronger the rest of the time. And, again, the problem is generally indicated at higher levels, not things like level 3 or 4 where casters are much less capable.
It is actually very swingy happening all over the place. In general it is not this class is good low, this one high, it is level ranges where they go from good to mediocre, to bad, back to good, to awesome ....
A Wizard is at or near the top at more levels than any other.
They aren't just better than me, they are better than me at all the things I built my character to be good at.
What is wrong with that?
Is your character viable? Then stop worrying about the other guy's character.