It's a shame, but I hope some other devs consider picking up development where the Perian guys left off. Perian is one of my favorite Mac utilities because I love using QuickTime for everything when possible. To the team, thanks for all your years of hard work and best of luck on future projects.
Perian just made open source codecs work with Quicktime's API. Quicktime X however demands much more that makes it impossible to do without rewriting all the codecs. Those same codecs are directly embedded in other players (like VLC) so it's not a big issue. Quicktime X is pushing towards hardware acceleration, better battery life (with a few tricks), and multicore processing that the old model just doesn't work with.
Right now when you run older codecs, Quicktime X actually runs a little host process in 32bit that actually does the decoding using a fork of the old Quicktime 7 code. Native codecs run in 64bit using multiple cores and hardware acceleration.