Let's be fair here: We have 2 playtests, neither of which showcase ANY modularity.
Well that's not true/fair...
"Modular" means simply that there are parts of the rules which can be taken out of the game (or into, depending how you see them) without affecting how the rest of the rules work. (However the meaning of "affecting" is not as obvious as it may sound.)
The Skills & Background system is modular. Take it out of the game and you don't need to change anything else, the game still works without skills (you just handle everything with ability scores at DM's call) or without backgrounds (you can use skills without using backgrounds, or neither). Furthermore, the classes and races maintain their original balance if you remove these.
The Specialty & Feats system is modular. Even easier than the previous case... feats are just add-ons, ignore them and the game is practically the same.
That's it however, practically nothing else is modular so far, although it's been announced that the Tactical Combat Module and Narrative Combat Module will be "modules" in this sense.
3ed
core were
not modular. You couldn't ignore skills or feats altogether, because that would have gimped certain classes. 3ed was the opposite of modular, it was "
organic", meaning that all the rules were interconnected. I actually liked that property, and I always say that 3.5 changes scarred that organicity somehow, which is why I went back to play 3.0. OTOH because of this property, it's hard to simplify 3ed, which e.g. makes it a pain for me to DM 3e games today that I have much less time, so I'm welcoming 5e modularity.
A new edition that allows one to play in the style of previous editions, is a better idea, and one I think WotC is aiming for. Set your expectations for a previous-edition-like-experience or a new experience and I think you won't be disappointed.
I agree on everything else you say.
Unfortunately I think in this they are already partially failing. 3ed and 4ed gamers may have no problems with 5e, and perhaps even AD&D fans. But I have serious doubts that OD&D/BECMI/RC fans (probably considered a small minority anyway) would be interested in 5e. Removing skills and feats is not nearly enough to provide an "old-school" feel... that is already impossible to achieve when you have for example 1st level characters that have even more stuff and abilities than in 3ed (even after you remove skills and feats), and genetically superior humans.