That shared prefix would be no worse tban a shared IPv4. Unless the shared prefix is per user and immutable.

For every ISP serving my area, it is indeed a "per customer, immutable" prefix. IIRC, some have a 96-bit prefix, some have a /64, but that's the kind of thing that a "maxmind" style database of prefix length per isp lets you nail down easily -- if those databases don't already exist today, they will soon.

It's easier for the ISP to do it that way.