It's much easier to deal with port opening or changing than it is to deal with a fragmented address space. NAT reminds me of the networking analog of segmented addressing on 16 bit 8086 processors, but worse as it is not systematic.

Opening a port through NAT on your own setup is pretty much the same process as doing it on any firewall. I'm not sure how the address space being fragmented changes anything in this case, the user must poke a hole somewhere somehow.

Try writing P2P software and you'll see it. NAT is like 16-bit segmented addressing. Go find some old 16-bit C programs and see what a "near" vs "far" pointer is and that whole mess. IPv6 is like the arrival of 32-bit real mode addressing with the 80386.

P2P software wouldn't work too well without fiddling with the firewall anyway, is what I'm saying. I don't think circumstances would have changed much if we'd adopted IPv6 instead of NAT. P2P largely failed to take off outside of piracy circles because centralization is more reliable and efficient, that's all.

And yeah, I've dealt with segmented memory models, in C and ASM. I don't think the situation is very comparable at all, nor are segments as bad as people say, but that's neither here nor there. I mean, FFS, all modern systems use an MMU that remaps memory addresses between processes anyway.