The web will not last forever. Better to build a foundation that is capable of bridging to the web if necessary than staking yet another UI layout engine atop a document markup syntax.
But even so, would you stake a current native UI toolkit to last longer, and provide more value, before that day?
There are good reasons to write truly native UI. Performance is very persuasive. Longevity, perhaps less so.
If you stake the basic ideas of the framework on the DOM, then DOM details will become implementation details which will become details that clients will depend on. And you'll have just re-written React but with even more layers.
It needs to be thought about in terms of first principles, ala SICP. There are basic primitives to be defined that are the "ground," and then it is built up from there.