How so? The same working group published e.g. https://www.rfc-editor.org/rfc/rfc1933, and it's hard to see how v6 could have been designed for backwards compatibility in ways that it wasn't already.

I've asked lots of people to describe a more backwards-compatible design, and generally the best they can manage is to copy the way v6 does things, ending up with the same problems v6 has. This has happened so often that the only reasonable conclusion is that it can't really be done any better than it was.