Ok, back from lunch. So, if you want to have a discussion (and not just argue)...
Since I've always provided justification for my position, as well as asking questions, you should have your answer by now...
This depends greatly on the style of game you are running IMO.
In a sense, yes, but as you can see below, not so much in the end,
If you mostly do dungeon-crawl, then the party starting the fight first depends enormously on how successful you are in infiltrating the dungeon (complex, castle, etc.) and how well the DM runs the "reality" of the location. IME too many DMs treat encounters almost like islands, where one has little if any impact on the other. Once the PCs fail to move to the next area successfully without alerting the entire complex (in a fashion dependent on the scenario of the adventure, of course), the PCs can often expect to be "encountered" as often as they do the encountering.
This is exactly my point, that is at least 50% of the time where the PCs choose their formation, meaning that they will protect their casters at start (and see below for how things develop).
Wandering monsters (if used, it is old-school for many games IME) will more than often initiate the fight.
Why ?
During travel, random encounters are nearly always initiated by the monsters, but this also depends a lot on the scene, what the encounter is, and how the DM sets it up.
Again, why ? You are assuming that PCs are always surprised by the encounter, but there is no reason for this. And it's not entirely up to the DM, how the PCs use scouts is important as well, and if I was a PC in a campaign where the DM constantly ignores all my precautions to spring surprises on me from the rear, I would be very frustrated.
Once more, in these cases, there is no more reason for the PCs to initiate than the monsters, and nothing says that even random encounters have to be encountered in a fight. If they are detected early enough by a scout, they could (and certainly should, in at least some cases) be avoided.
If the PCs start the fight, they usually have only one point of entry, but often need a martial to watch the rear because very often the enemy will move to flank and attack from behind if possible.
Again, you are assuming that the fact that the PCs have the initiative will mean that the enemy can always counterattack from the rear. Why ? And you seem to be under the constant assumption that the enemy outnumbers the PCs, which is certainly not the case all the time.
While many monsters are not very intelligent, the average INT for anything CR 1/2 or higher is 10, and it rises with CR. So, with the exception of super-low CR creatures (many are beasts and such), most foes WILL be smart enough to employ tactics that suite them best in defeating the party. I'm not saying those tactics have to be brilliant or anything, but flanking and rear attacks are not hard concepts when you have numbers.
Again, why should the enemy have more numbers. In 5e, in particular, the encounters are often evenly matched or even in the PC's favours because of action economy impact.
Given that most parties have 4 PCs, in general if you want an encounter to be any real challenge at all, CR monsters half the level or lower of the PCs will at least match their number, of often outnumber them.
Again, why are you systematically using small CR monsters ? Nothing implies that anywhere in the rules.
Eve BBEG encounters will have minions to support the BBEG by harassing fringe party members when the front-liners engage.
Again, not necessarily.
FWIW, as DM, I also try to have my encounters be a mixture of CRs (if it makes sense to do so), and have smaller CR minions in larger numbers because with bounded accuracy they can make a difference.
And that is your choice as a DM, but nothing either in the rules or in the published adventures make it so that all the encounters or even the majority are built like that. Actually, looking at published encounters, these are by far the minority.
At any rate, casters in the rear is a mistake more often than not, they should be center of the party, or they are subject to all kinds of nastiness. Depending on the encounter set-up, even then they aren't necessarily "safe" by any means compared to the martials. More intelligent foes will attack casters with range, numbers, flanking, or whatever. If martials are engaged, they have to disengage to help the caster or wait until they are otherwise free to do so. Your "rear-guard" might be a ranged martial or mobile one who can move up if it is safe to leave the casters behind.
The problem is that you are assuming that monsters are always intelligent, always have superior numbers, and that the PCs are globally stupid and don't take into account the monsters tactic, who can always outflank and take them from the rear.
These are encounters from Rime of the Frostmaiden:
There are quite a few with unintellignet beasts, in most of the cases the PCs outnumber their adversaries, and nothing says that one party will surprise the other or have a tactical advantage that allows him to outflank/surround the other. It entirely depends on the circumstances. And there are few cases of "boss + minions".
I understand that's your favourite style, and I'm fine with it for some encounters, but again it's not the general case, and as a player I would be very frustrated if all the encounters turned out into a DM vs. Players situation where the DM is basically there to explain to the players that he can outthink them tactically using monsters designed for this.
