I think a more practical approach is to focus on a generic enough UI toolkit meaning detached from any specific tech stack that will give a feeling of continuit

I think a more practical approach is to focus on a generic enough UI toolkit meaning detached from any specific tech stack that will give a feeling of continuity to customers as they move from static pages and SPAs and different flavors of ideas that IT tried along the way. One SPA to rule them all is ridiculous as it doesn't work that way with scale as you have independent teams working on various projects that will be hindered by a single approach in that everyone will have to understand x approach and also have stupid meetings about it.