It's not that it wasn't considered. The biggest problem with interop between v6 and v4 is that you can't really do interop between v6 and v4. The v4 header only has 32 bits available for the dest host, so there's no way to specify which v6 host you want to send packets to.
Unless you count NAT64-like solutions or 6to4-like solutions, both of which do already exist.
And IPv6 still has the same shortsighted flaws for futureproofing as IPv4; It lacks extensibility. Sure it looks infeasibly big now, but they keep saying that and then we find we run out of space. It wasn't that long ago when a terabyte was considered unbelievably big yet now computers routinely come with drives of such capacities!
It does lack a way of expanding the address space, but we'd need to actually run out of space first for that to be a problem, and 128 bits really is a lot. 1 TB drives and v6 are in completely different ballparks: if v4 is 1 TB, then v6 is 80 million billion yottabytes. There are 300 million /64s available... for each person on the planet. And each /64 has essentially no limit on the number of hosts it supports. I could understand an argument that each person might end up running billions of computers (which would be no problem at all), but a quarter of a billion networks? Each?
And that's just using the 2000::/3 space. There are five more unused /3s available, so we could do it all over again five more times (presumably with smaller-than-/64 subnets) before actually running out.