Im bummed about the modularity too (though I never thought 1E next to 3E across from 4E PCs at the same table was ever possible). Modularity is still on the table and likely a stronger tool int he incremental tool kit. They just haven't had to reach for it yet.
I don't see how that's even remotely possible. The game, as it exists, cannot accommodate a character anywhere near simple enough to satisfy a 1e/2e fan; the closest fit, Champion, is and remains awful, and the playtest has done little to nothing to fix this problem. Conversely, with the slapdash """balance""" of 5e and dearth of meaningful choices whether in or out of combat, plus the bizarre culture of play that ensures 5e skills are incredibly narrow in scope despite the text not actually doing anything to foster such a notion, it's essentially impossible to get an experience even distantly like 4e.
"Modularity" in any meaningful sense is dead, and no amount of iterative change will resurrect it. You would need to rewrite core math components--not to mention a MUCH bigger overhaul of classes and (especially)
spells than WotC is willing to consider--to even attempt it. At the very, absolute least, no amount of iterative improvement can add strong tactical play to 5e, and no amount of iterative improvement can
remove the abundance of magic in 5e. Those two things nix 4e-style and old-school-style right out the gate. The only way to get them is for the DM to extensively house-rule, at which point it isn't even 5e doing it, it's the DM creating her own game
built off of 5e.
Which, you may note...is what a lot of the early/prominent podcast groups did. Many of them either built their own game, or switched to a game someone else made that better fit what they wanted to do. Funny how often that's happened.
You need to reset to add advantage/disadvantage? Buddy, you aint convincing anybody.
Certainly at this point we would need to reset to
fix how they used--or, rather,
abused--Advantage in 5e. A fault I predicted before the game was even published. I could see the writing on the wall. They would over-use it to a crippling degree, because it was simultaneously a weapon of both first and last resort. The only part I didn't predict was how early they would start adding ways to break Advantage wide open (e.g. Elven Accuracy's super-advantage.)