...all of which structures you'd somehow need to synchronize writing to, to keep them accurate and fresh. Global write-locks do not scale, no matter how fast it might be in the toy case.
Indexes in postgres point to the page, not the tuple. The VM keeps two bits per page: "does this page have any 'dead' tuples?", and "does this page need to be touched for xid wraparound?"
Vacuuming doesn't touch a page unless the VM indicates it's necessary.
Except for BRIN.