Ayep.I suspect that the "obvious" lesson to be learned from 5e is that the game should not be unnecessarily complex in regards to its subsystems.
Like it or not, far too many D&D gamers just aren't interested in how other games solve issues.However, I'm not sure why 5e should be the primary teacher of this idea when - much as you say here - there are so many other games out there, before and after 5e, that do a FAR BETTER job than 5e at designing more streamlined, less complex, and more robust systems than 5e.
Being able to say "but AD&D fixed it" or "it works without a hitch in 4E" or "just look at Pathfinder for a much better solution" carries much more weight than pointing to OSR or Apocalypse Now or Call of Cthulhu. While this is partly unwarranted, it actually is partially warranted. D&D needs to solve a particular set of questions that other games can just drop.
I might not like everything about 5E, but i consider it undeniable that its designers managed to truly get rid of the 3E crud and many darlings were killed. 5E really represents a very impressive effort in pleasing your customers while actually not listening the the demands that existing detail and structure "must" be retained.
Sure, the game erred on the side of simplicity too much, but still: PF2 would have been a vastly better game had Paizo bothered to check up on the competitition... and made that show in its own rulebook.
Absolutely. And when it does I'm there to point it out. (For instance with its legacy crud about hand usage, spell components and object interaction. When you read those rules you get the distinct impression you're no longer reading the easygoing game 5E is elsewhere)5e arguably preserves a lot of needless complexity itself while also handwaving its complexity with its "rulings not rules" mumbo jumbo.