The thing is, while this is a valid question (and despite what I say in the beginning, I think there is absolutely some modularity to 5E), it really wasn't what I had in mind. I was much more interested in discussing what a 5E that could reasonably emulate all editions within it would look like (which is what Monte Cook and the early design goals of the playtest indicated). The fact that most of the pages discuss not this but prefer dunking on the thread by saying "5E IS modular, duh" or going off onto a tangent about quality is a bit sad (also, once again, the point about OSE's model of genre rules is completely ignored!).
My thought is that it would have been a logistical nightmare.
To start, you'd have to look at each edition and ask "what isn't being emulated in the current core rules?" The follow up questions are then "is that feature even compatible with 5e" and "if allowed, would it be fun?"
Take BECMI's race as class. It would be possible to make a dwarf, elf, and halfling class for example, but I don't see what advantage they would have over the normal race/class system. At best, we get a decent gish class. But more than likely, we have a new class scarcely better than a dwarf fighter with a lot less customization. Yet race as class is one of the THE defining differences between Basic and Advanced D&D.
You can do the same with nearly every edition: would a class you can only take after a specific multi-classing combo be fun enough to emulate the 1e bard or 3e prestige class? How do you balance 2e-like specialty priests? How do you even begin to fit an alternative ADEU system into the current 5e classes without basically rewriting all of them?
And that's assuming the core rules. Don't even think what kind of nightmare replicating 2e Player's Options would look like!
I think at the end of the day, they tried to get as much as they could fit into the core rules, but I think people were dreaming of things that are impossible. You can't make an edition where a 4e warden, a 2e specialty priest, a BX dwarf and a 3e spirit shaman could ask play at the same table and feel like their respective editions. However, you could make a oath of ancients paladin, a dwarf champion fighter, an oath of shepherd druid and a cleric of whatever domain you want and all feel something like those older editions in the "if you squint hard enough" sense.