Because I fully agree with you. 5e is, for good and for ill, a better-made version of everything 3e tried to be. And one of the things 3e tried to be was "ruthlessly optimize yourself, your own contributions, above all else. You are not a team. You are 4-6 individuals who happen to adventure in the same place at the same time."
My apologies for suddenly jumping into a discussion like this.
I am a long-time lurker, albeit I posted here a long, long time ago.
I was lurking the thread, in fact, searching for ideas. The topic is dear to me because I am working on a 3e/PF homebrew and tinkering with a subclass of the Cavalier that is closer to the 4e Warlord. While 4e is not my favorite, I think plenty of its good ideas can be applied elsewhere. I also think that a good game has design space for people of many different tastes, so I am also doing this after endless discussions with 4e players expressing their love for the Warlord - I think they also deserve to be happy, so to speak, and I think their complaints ITT are mostly legit.
Additionally, the concept is far from being alien in 3e - there, you could easily create a Marshal (Miniature Handbook) with a Warblade-type maneuver mechanic (Tome of Battle), refreshing limited to White Raven and calling it a day. Perhaps one could add a mechanic that allows to switch between Marshal auras and granting teamwork feats (PF 1e, Advanced Player Guide, see also Cavalier in the same book, especially Order of the Dragon) to nearby allies.
Nonetheless, the reason I felt compelled to answer is the quote above, and other comments in the thread which are, in my humble opinion, needlessly bashing 3e with little knowledge of how a good 3e game worked.
You can totally be a team in 3e - in my experience, genuinely skilled people build up their character to do so. Besides the obvious buff spells which can be selected to cover corners, (any spellcaster) or buff mechanics (auras, bard), Rangers granting flanking with hits (ACF), energy drain, or a hexblade curse, or the infliction of the shaken (Fighter, Barbarian, Rogue) and sickened (Rogue) condition to a target before using a big gun spell are a thing, and so is maneuvering targets into a spell area of effect, or even throw smaller allies! Item use is also essential, like a well-placed tanglefoot or atramen oil (OP!). A well-shaped Wall of Stone can solve many issues for a Charger melee, which will then one-shot the target and spare a lot of spell slots.
Simply, 3e "combos" and synergies are not immediate and intrinsic to the powers and spells used. No "pre-made," so to speak. They just do what they do because 3e is built to first describe an action proper of a fantasy world and try to write down a mechanic that is the least dissociated possible. It is up to the players to decide how to use them, often on the spot and in specific contexts.
Yes, when you are high level enough, you can sometimes dispatch on your own certain enemies. There is nothing wrong about that because it's a prerogative of very powerful characters. If you started at level 1 as you should, you ultimately earned that! You must be careful though, because those enemies could do the same to you if they are powerful enough. And that's fine - high-level games shouldn't be just low-level games with bigger numbers.
5e is not "a better 3e". For many 3e players, 5e is 3e without most of the things that made it interesting. Sometimes, with glaring holes.
I still appreciate some change - as an example, how wands work.