[lmm]

> Google could put a tree-like organization of docs on the sidebar, make the search a bit comfier, and make draggable blocks, and get 80% of the Notion users.

Technically they can. Organisationally they can't.

> I guess someone could build a browser extension that adds that UI to Google Docs, or eventually I'll go and do it myself.

You won't though, or at best you'll make a rough-and-ready version that works for you. Polished stuff doesn't get created without a business model, and you can't make a business model out of a browser extension that messes with a third-party site, not these days anyway.