Hey there, core contributor here! Starting with the last one first, since that's the easiest - VJSv10 is basically a completely new player, so no backwards compatibility planned (think Mac <=OS9 vs. OSX). We're aiming to port some of the popular plugins though and have discussed other things like migration guides and the like.

For the primary question - this is a tough one, specifically because v10 is a completely new, ground up architecture. Part of this will be feature parity - v8 does many things/handles many cases that v10 doesn't do yet. That may seem like that is an unfair comparison, and, in some sense, that's true. However, this is in fact part of the ethos of our new architecture: by building a highly composable, loosely coupled player framework with well defined architectural "seams"/contracts, you can more easily pull in "all and only what you need for your use case" (a phrase I've been bandying about). While v8 allows for some of this, it's still much harder and you still end up pulling in stuff you probably don't need for a lot of use cases.

Another one is the UI layer - v8 ended up building an entire component implementation. At the time of building, it kind of had to. v10, on the other hand, can "stand on the shoulders of giants", building on top of e.g. custom elements, or React, or any future frameworks we decide to target (and our architecture makes that comparatively easy as well).

I do suspect that once we hit true feature parity, the numbers will be much closer for "the kitchen sink." The thing is, few people (if any) need the kitchen sink.

Thanks for the tough question!