Instead, Apple killed Shake and let big studios buy the source code from them so they could continue to use it.
There is a massive difference to Motion and the highend compositing apps like Shake, Nuke, Fusion, Softimage(Illusion+Matador=FXTree), Houdini(COPS), Flame, Toxik(Autodesk Composite). After Effects and Combustion are time-line based like Motion, but Motion doesn't begin to compare to them. Even Blender has fantastic compositing tools.
Shake was serious business. It's last well known big project was 'The Dark Knight' which Framestore CFC mangled a 64bit wrapper around it to better manage the 8k-frame workload. Shake was ported to Intel, then killed off, with no intention to take it to 64bit. I don't think you can buy the source code anymore.
They've done it with Digital Domain and Nuke (compositing), Weta and Mari (3d paint), and now Sony and Katana (lighting).