You may tell me that a warrior would do warrior stuff, a healer would heal or a rogue would do trickery stuff at 100% efficiency, but there are many other classes that could do that at a 90% efficiency. So yeah, you win, there´s a difference, there are classes better suited for a particular role, but that does not mean that plenty other builds that could cover for it if you do not have/want that class in your party in 5e.
Um, gotta disagree with you there. For most of the skill based examples you give, sure, yeah, not gonna debate you on that. 5e leveled out a great deal of the skills, experience and made a lot of stuff flexible or same in that regard. However, you lost me at the "there are many other classes that could do that at 90% efficiency." Nope. Just not true for a great many things.
Ranger never gets access to a wide range of spells the cleric does. Need remove curse? Best of luck to ya. Need a raise dead? Never gonna happen. The ranger literally caps at spell level 5 with a far more limited spell list, compared to a huge spell list for clerics and top out at of course level 9 spells. How does the ranger then fulfill the role of "healer" at 90% efficiency? About one of the only class relations I would agree with that statement remotely is bard, wizard, sorcerer, warlock, but those are all magic focused classes anyway with variations. But can any other non magic class cover the role of a "wizard" at 90% efficiency? Again, it's a no. Not gonna happen. No class that doesn't max out at lvl 9 spells is going to even hold a candle to a class that does. A paladin may be able to heal but again, not like a cleric. A spell sword might be able to cast magic but not like a wizard or sorcerer. Anyway, the point I wanted to make was already made. Skills, yes that's one thing (except I still want a rogue for the specializations for rogue like things.) But saying one class can be adapted to be 90% as good as another class at something I simply see as just not true. Conflating various mostly non class skills & backgrounds with actual class skills and progression which is NOT the same. And I'm not here to argue and you clearly know your stuff regarding the game but I feel that was a pretty generous over exaggeration.
And as far as what works in a table top game, I don't really see that as incredibly relevant because most DMs that aren't shooting for a TPK are going to be nice and adjust for some of the weaknesses in the party where as hard code is unlikely to do that. Don't have the speak with animals? Well I guess you aren't doing that then hmm. Want to get through this door? Oh well, you're going to need a rogue with specialization for this best item in the game...
To me though all of this back and forth about CAN you do this or that with a smaller party is moot though because clearly there are lots or even more people that would love to at the very least have the option of a 6 character party, which has been the standard in basically all of the previous games from Icewind Dale to BG to Neverwinter and some of the games that have come after like Pillars, Kingmaker etc as well. If it ain't broke don't fix it, and certainly don't take away 2 characters and say, well its fine, you can just try to squeeze all that down into 4 characters instead. Ok but, lets make that an option. 6 SHOULD be the STANDARD, not 4. And everybody who is ok with 4 can rationalize it in their own way but I'm just going to keep disagreeing and pointing at the boxes of all the originals, and the fact that they have.... 6. And lastly I'll say it again, personally I'm sick of 4 player max co op games anyway honestly. Which this isn't just any game, it's a D&D game that shares the name with the game that started it all, the original 2D isometric classic. Some things are bound to change but party size being capped at 4 shouldn't be one of them. Swear to goodness if it's capped at 4 it's going to feel like some kind of D&D divinity to me and not actual BG like it should be imo. I guess we need options to make everyone happy as it seems relatively 50/50 or if this thread has any bearing like actually more people would prefer 6 over 4.
Side note: My favorite trilogy of books is the Dragonlance - Tales of the Lance. Wouldn't have been such a great trilogy if they couldn't have ever assembled more than 4 heroes at a time... >.>