I'm not part of the TipTap team but I think I can help answer this.

Think of it like this... you're basically only coupling yourself to ProseMirror. That's the schema you're storing in your database. TipTap is a set of libraries that you can use for your web app etc... but if you ever find it insufficient, you're free to role your own thing.

ProseMirror has been around for a long time and is unlikely to be going anywhere, but having a company with VC funding using it as its base helps ensure that if ProseMirror got abandoned, it'd likely get picked back up by TipTap or some other interested party.

Even if you're building your own editor, unless your editor has a need that ProseMirror's schema doesn't account for or solve it makes sense to use it.