All of those are problems which extensions already mostly solve. Firefox has tree style tabs, sideberry and tabstash. You can hide the horizontal tab bar with css. You can create multiple profiles in Firefox, and you can open separate isolated windows for each. Notes depend on your chosen software, but simple browser-only extensions exist. I don’t care about mini windows; that is a non-issue already solved with tab mgmt keyboard shortcuts.
Yes, but those companies are poorer, and have lower tier legal teams.
Das capitalism baby!