I think calling Ubuntu Desktop a newbie distro is somewhat short-sighted; it tries to be a distro for all people who want a working desktop system. And that includes a lot of application developers, sysadmins, scientific researchers and artistic people who don't want to spend their time building/fixing an OS.
Historically speaking, it's others that pushed their own technologies instead of upstart, not the other way around.
And AFAIK other distros don't have anything like Ubuntu Software Center. Linspire had CNR, but that doesn't exist anymore. Are there any others?
In case of GNOME Shell & Unity, I think those started around the same time, and developers from both projects did meet even, but maybe their ideas were too different (although you can still see the common ideas discussed during that meeting too)?