This is absolutely what I had running around in my unconscious without knowing it. There is a creative power in having a baseline that doesn't have dragonborn (just using that as a poster-child for new school fantasy species explosion), and then having whatever weird stuff show up occasionally. Dragonborn (or draconian) crash landing from a space ship (or Spelljamming vessel)? Cool! Every new race having a population on Oerth just because it exists as a player option in 5e? Boo, hiss, boo!
If I had my druthers (what are druthers, and why would I want them?) every setting would be rebooted to its original recipe for 5e, and then they'd give advice for advancing the timeline in a variety of ways (including for each edition's version), including references to other books from older editions that are most useful for those eras. The official adventures would also follow this paradigm. So all Forgotten Realm adventures are based on the Grey Box, and then have a page or two in an appendix for setting them post Time of Troubles, post Spellplague, post Second Sundering. Sure, they could make a bigger appendix for setting them in the latest published era. But really, this would be nothing but win for sales. They seem to imply that they are going for timeline agnosticism in the FR adventures, but then they bake-in the 5e elements in such a way that it's a major hassle to take them out. I'm never going to buy most of the Forgotten Realms adventures in 5e because of this, whereas I would likely buy several of them if I didn't have to try to revert the timeline a couple hundred years to make them useful in game.