They took much more on with IPv6 than IPv4 replacement. The spec goes much deeper than IPv4 did, replacing ARP, DHCP, etc. It's a product of its time, including a lot of over-engineering by committee. Many of the problems they tried to address didn't pan out to be real issues. You can read the RFCs and compare.
IPv4 w/ more bits is a lot more simple. Yes, older network gear wouldn't deal with it well, but that's not a real issue today because that same network gear supports IPv6.
Buuut, one of the biggest problems with app-level issues is just that the app doesn't bother dealing with IPv6 addresses and AAAA records. It would be the same issue with an imaginary IPv4*2.
No kidding - they got rid of dhcp and more but it’s a nightmare getting networks to work with just ipv6 concepts- everything from provisioning phones (dhcp options to push config / NetBoot stuff) and more. Layer in privacy extensions, renumbering on uplink wan flapping (slow too - the failover is pathetic compared to NAT wan failover) - icmp traffic differences - firewalls need to be much more careful with ipv6 and related protocols because things can easy break (it’s fragile) or you create risks. Even min subnet sizes mean crazy 2 node subnet sizes (think isp and cpe management subnet for a customer). Also curious why not 48 or 64 bits or 96 bits? 128 bits is ludicrous
128 bits was probably picked to give 64 for "MAC address-based locals" and then the reasonable thing is to have 64 more bits on the other side, if you only had 32 you're just IPv4 with more steps.
Layer 3 exists as a layer of routing and aggregation on top of layer 2. Aggregation necessarily consumes address space, so L3 needs to be bigger than L2 to accommodate the full L2 address space. The L2 address space is 64 bits and the next power of 2 up from 64 is 128, so here we are.
96 bits would probably be enough too, but having large subnets has a few benefits -- it allows for securing NDP by using the extra space for a cryptographic key, and also it makes it much, much harder to scan for active hosts from outside the network.
Plus, can you imaging the wailing and teeth gnashing we'd be getting if v6 wasn't a power of 2 bits long?
IPv4 w/ more bits is a lot more simple. Yes, older network gear wouldn't deal with it well, but that's not a real issue today because that same network gear supports IPv6.
Buuut, one of the biggest problems with app-level issues is just that the app doesn't bother dealing with IPv6 addresses and AAAA records. It would be the same issue with an imaginary IPv4*2.