Python 3 came from a good place, and it definitely fixes many problems that sorely needed fixing, but it was doomed to failure from the start (and many developers said that in 2008 already). For all intents and purposes, Python 3 is pretty much a new, separate programming language. Sure, it's very close to Python 2.x, but you don't have out of the box retro-compatibility so that pretty much kills it right there. Pyth…
I don't know if that's necessarily a foregone conclusion. Scala releases aren't always/usually backwards-compatible, and you'd be hard-pressed to find people still using much older versions.