If you want to run a single massive scale network sure. One of the costs of scaling that the majority don’t see
No reason you can’t carry IPv4 over any protocol you want. Multi tennant vxlans can carry whatever you want over your base network. Maybe an IPv6 underlay makes sense there, doesn’t really matter
Mostly they do carry IPv4 over IPv6 if they have vaguely newish equipment, otherwise they carry IPv6 over IPv4.