Routers don't care about the TCP header though, they only care about the 20 bytes of IP header. The interfaces on the router also care about the 14 byte MAC header, but that's a separate step.
That said I agree that the world likely would have sucked it up and just gone with the 64 bit addresses, but there would have been a whole lot of grumbling for decades about the memory use. It's hard to imagine these days, but back then memory was outrageously expensive and enormous amounts of engineering went into minimizing memory use wherever possible. Having these addresses where we wouldn't even touch half of the bits for decades would have been unpopular with a lot of people, even if he would have been praised for being so forward thinking in the end.
On the other hand, the transition to IPv6 does give us a chance to fix some of the longstanding defects in IPv4 that would otherwise be baked into the protocol until the end of time, so it's not all bad.
"Having these addresses where we wouldn't even touch half of the bits for decades would have been unpopular with a lot of people, even if he would have been praised for being so forward thinking in the end."
Worse perhaps, people, algorithms, hardware, ... will all start to assume those bits are zero and start optimizing for that. We have seen this happen repeatedly throughout history (eg. Lisp pointers would stuff tags in the "spare" bits in addresses, making implementations non-portable).
These are all hard problems, but the lesson I take away is to
1. Always have a plan for evolving the design/protocol/API/...
2. Aim for a watertight design that supports 1. (Example: TLS 1.3 puts random stuff into reserved fields so nobody can assume they are zero).
3. Really stretch your imagination about future usages (assume 50+ years)
4. Have a ridiculously comprehensive test suite for both clients and servers
It seems clear that in terms of lifetimes, hardware gets used much longer than expected, software even more so (Y2K anyone), but protocols/formats/specs/... literally never dies
cisco was founded five years later, in 01984, at which point regular ram cost on the order of a dollar a kibibyte, so the content-addressable memory you need to accelerate a router was maybe ten or twenty dollars a kibibyte
would people complain about the ip headers having unnecessary junk in them? certainly, but there is plenty of that in the actually adopted protocol header design too, so clearly it wasn't a showstopper
That said I agree that the world likely would have sucked it up and just gone with the 64 bit addresses, but there would have been a whole lot of grumbling for decades about the memory use. It's hard to imagine these days, but back then memory was outrageously expensive and enormous amounts of engineering went into minimizing memory use wherever possible. Having these addresses where we wouldn't even touch half of the bits for decades would have been unpopular with a lot of people, even if he would have been praised for being so forward thinking in the end.
On the other hand, the transition to IPv6 does give us a chance to fix some of the longstanding defects in IPv4 that would otherwise be baked into the protocol until the end of time, so it's not all bad.