Ummm, not really. His track record was rather spotty. I mean, that was his whole joke about him, this beloved geek god, but his stuff always ended up getting cancelled and not reaching that critical mass to be financially viable.
Buffy was a huge smash, which allowed him to do Angel, which did okay, then he did Firefly, which was cancelled, then Angel was cancelled, then Buffy was cancelled. Then, he tried again with Dollhouse, which was not great, and was cancelled.
I have his entire run on Astonishing X-Men, the first arc or two were awesome. But it got rather strange around the Danger storyline, and it just didn't end well.
I'm pretty sure the Serenity movie didn't do all that well at the box office either.
He does great stuff, but it wasn't until The Avengers where he reached critical mass. Now, he's being given carte blanche, which is probably great, because I tend to think that it's outside interference, handing him notes and such and telling him to do things a certain way, that just doesn't work. All the Buffy and Angel eps he did were great, you can always tell what's a Joss ep and what's not.