At no point did I restrict it to PHB only. I want all the backgrounds. Some of them do not include any cultural points and I have asked repeatedly how you intend to add culture to them.
All backgrounds are culture. Albeit some cultures are international, like Sailor, Farmer, etcetera. Many nations have some comparable way to supply these backgrounds.
The fact that the core book contains the majority of the list of ones that your plan does not work with is not my restriction at all.
Notice that the Baldurs Gate adventure setting REWRITES the Player Handbook backgrounds to represent more accurately the local culture of the City of Baldurs Gate. This city is one of the more prominent cities in the Sword Coast region, and has an international flavor with a diverse cosmopolitan urban culture.
So, an elven acolyte in what you are saying will lose all of their elven culture feats, but can sub in some from backgrounds.
Not sure what you mean by "culture feats"? Do you mean Xanathars race feats? Do you mean Player Handbook race features like Elven Weapon Training? If the latter, the UA Trance Proficiencies feature seems to replace the PH Elven Weapon Training feature.
Background can supply various cultural features, including learning to use a certain tool, knowing a certain language, and so on.
Which you can already do -"Custom Background" is part of the core rules.
My point exactly. Several official WotC settings are already taking advantage of customized backgrounds, to provide setting-specific features.
So background remains the same power, and the elf race loses power. That was one of the options I put forth.
I feel the races in the Players Handbook are unequal to each other. Some subraces are more powerful and some are less powerful.
So far, the recent races that use the format update feel equal to each other in power. I am very happy with this amount of design space.
When 5e updates the Player Handbook, such as for 50e, anything that is removed or relocated will be filled in with something else to fill out the design space.
So there is no loss in overall power.
Apparently, a 50e elf Wizard can use the new Trance Proficiencies feature to wield a longbow. But the elf Wizard might want to spend a background to gain this proficiency permanently. In any case, the elf features will be sufficiently powerful, and a cultural bow proficiency worthwhile for a Wizard to invest in.
So, that isn't compatible with existing characters, so it's not what they are doing. Easy peasy.
In terms of overall power,
50e elf + background ≈ PH elf + background
The designers want backward compatibility. So, a player can choose whether to build a Tashas elf or a 50e elf. Each variant will be comparable in power. No problem.