I initially viewed the introduction of Feats into backgrounds with dismay, as I very much liked that Feats were opt-in in the 2014 rules. I don't like Feats, and I did like how simple character generation was in 2014. It felt like a step back to the super-involved chargen of 3e and 4e.
I am somewhat mollified to hear in the video that pure ASIs are still included in the game, so that you can opt-out of Feats as part of character advancement if you want to. They just wanted to beef up the backgrounds with mechanical weight instead of the more nebulous ribbons of 2014 Backgrounds. And I can live with that.
In the Gen Con TV retrospective on 5e, Mike Mearls made a very astute observation that I think needs to be considered when looking at their decisions for the 2024 PHB. And that was, while its said that most players only buy PHBs, in actuality the vast majority of players don't buy anything. The show up for a game, borrow someone's PHB to make a character, and that's it.
Everything I've seen about the PHB indicates that, while we who buy all the core books and lovingly pour over each page will still get all that we want and need, the design seems very much geared towards those who won't own the books, but rather will be making only periodic reference to it at the table. The Rules Glossary is one example of this. The Character Class and Subclass summary page is another. And I think moving the Custom Background rules to the DMG is yet another.
For regular players, Custom Backgrounds are virtually a fait accompli, but for a new or casual player, devoting the PHB's limited space to only the provided Backgrounds makes things clearer and more straightforward. And not only would making a Custom Background generally have to involve the DM anyway, but creating Custom Backgrounds for one's own homebrew is also something DMs will likely like to do. Seen from that lens, rather than the Player vs. DM power dynamics that are a favorite topic of forums like this, putting Custom Backgrounds in the DMG make eminent sense.