How does your backwards-compatibility dream handle ASICs built to handle only IPv4 and shitty middleboxes hard-coded to drop or -worse- mangle any IP traffic that doesn't fit a particular subset of what's permissible for IPv4?

Google and other big players go to huge lengths to build new Internet protocols on top of UDP because enough of the internet drops or mangles anything other than TCP or UDP that it's effectively impossible to use anything else on the Greater Internet. IPvNext by way of backwards-compatible IPv4 was (and continues to be) no easier than doing something that's backwards-incompatible.

As a bonus, doing the backwards-incompatible thing bypasses all the bad behavior of existing shitty middleboxes and crummy ASICs.