Here's what I found.

Why does Totem need to be replaced?

Totem is still a GTK3 app and is unmaintained (in part due to a crusty codebase), seeing no major development in years. Replacing it with a modern GTK4/libadwaita app designed to use modern technologies and meet modern needs has been a “high priority” for GNOME.

In this case it's more of a switch away from the last cool new thing. Totem (like Music) was built around a media library navigated from within the app. By default Totem doesn't even support opening videos from the file manager, which is something you would probably expect of a video player. It also crashed for me when I tried using it as intended so I'm not surprised to see it replaced by an app that really is just a video player.

That said many apps get replaced not for feature reasons but just by being GTK3, and they tend to get replaced by their own forks to GTK4 (such as the upcoming replacement of Evince). Why their devs choose to upgrade toolkits this way I cannot say.

GNOME mostly abandons their old apps. However, in some cases, the Xapps project has taken over these older code bases.

https://linuxmint-developer-guide.readthedocs.io/en/latest/xapps.html

I don’t think they’re usually abandoned. At least not right away. But they rarely still get feature updates. Mostly just bug fixes. Not sure if it’s just different developers not wanting to stick to the same project of someone else’s code or what.