How sure are you that 40 bits is a good number of bits? What's your justification? It takes over 30 years to deploy new bits, so you have to be really sure before you start that effort.
40 bits would've bought us a lot of time and would've kicked the can down the road several decades. People from the future would be much better equipped to design a new protocol because they understand their needs better.
How many decades? If several means six, we're already halfway there and at 50% deployment. What's the point of designing something you know will be obsolete the moment it hits 100% deployment?
It really isn't. I mean, at a very basic addressing level, sure, IPv4 addresses and IPv6 addresses are just a sequence of octets. But at a network infrastructure level they are quite different, and IPv6 is (conceptually) designed for large-scale, hands-off deployments. See: SLAAC, NDP, DAD, GUA/ULA/link-local address scopes, unicast vs. multicast vs. anycast, etc.