How would changes not have been required in the ISPs? An IPv4 router wouldn't know what to do with a 4.4 packet. At best, it'd route it to the wrong place - 1.2.3.4.5.6.7.8 and 5.6.7.8 are totally different hosts that may well not even be on the same continent.
Additionally, the only reason so much code had to change for ipv6 is that Berkley sockets is a terrible, terrible API that has abstractions so leaky they might as well not exist. Sure, in other APIs (what few exist) low-level code had to be rewritten somewhat, but that's going to be true for any protocol change, because that's kinda what change means.
I think you have missed that a IP4.4 packet would be a valid IP4 packet. The first 4 octets of the 4.4 address are where IP4 expects them to be. The router at this IP4 address needs to understand IP4.4, but routers before do not. The additional octets are smuggled within the IP4 options header.
I didn't claim it to be a new idea - I asked why we didn't do something simple like that (as the solution) instead of all the expensive complexity of trying to upgrade the entire Internet to IP6 over multiple decades.
That was my point: we did. It turns out people prefer to deploy v6 natively.
(Also, I don't think it's fair to call it simple. Many of the things we've done to deploy v6 are things which need to be done to deploy any IP protocol with bigger addresses than v4. If you count those things against v6 while ignoring them for any alternative, you aren't doing a fair comparison.)
Additionally, the only reason so much code had to change for ipv6 is that Berkley sockets is a terrible, terrible API that has abstractions so leaky they might as well not exist. Sure, in other APIs (what few exist) low-level code had to be rewritten somewhat, but that's going to be true for any protocol change, because that's kinda what change means.