alt Hacker NewsThere were backwards-compatible protocols proposed, such as EIP, but the committee chose a backwards-incompatible protocol for v6. Their assumption was that v4 would run out of space in a single-digit number of years and everyone would be forced to migrate. The past 30 years have shown that not to be the case.
They went with SIPP, which was one of the backwards-compatible options. It should be kind of obvious from the vast number of backwards compatibility methods available in v6 that v6 is actually backwards compatible... but for some reason a lot of people either refuse to believe this or have double standards around what counts as compatibility.