I think you can both be disappointed in the fact that he's going to host the late show so no longer be satirical Stephen and not think he's a sell out. Disappointed /= thinking he's a sell out. The two ideas aren't married to each other. I'm quite disappointed. The Colbert Report is unique and clever, and I enjoy satire far more than either straight comedy or straight, educated take-downs. I think this is a huge loss, regardless of when he decided to do it because, yes, he wasn't going to keep doing it forever.
But, I also have no interest in talk shows: daytime, late night, any of them. And I'm only vaguely interested in seeing what Stephen's like as host of the Late Show. If he makes a Tolkien reference every day, I may be there, but that's highly unlikely, and the format in general isn't one I'm interested in watching.
So, will he lose me as a viewer? Probably. But not because I think he's a sell out.