> In theory IPv6 solves the problem by putting all of this under a single prefix (fc00::/8) and asking nicely that implementation/admins add 40 random bits to form their prefix. Which is plenty to prevent (almost all) eventual conflicts.

IPv6 solves this by making it easy for everyone to get their own 200::/3 address so that when two companies get together they have completely different /48s (or whatever).

> […] but then the same could be said of IPv4 where adding entropy to 10.X.X.0 is the usual recommended way of doing things.

There has never been a recommendation to add entropy to 10/8 addresses. At least I have never heard of such a thing. Can you provide a citation for this (alleged) recommendation? Is there an RFC of some kind?