I don't know if Kris could do a concert all by himself and pull it off. I enjoy what he does and have the entire season; I can play the piano and the guitar but I can't sing much for shit, so I always respected how he hung in there (and eventually won) a show with more talented singers using those skills.
The problem I would have is, he isn't the best at what he does. There are other guys, such as Jason Mraz, or Justin Timberlake, who excel at playing guitar/piano and singing, and do it better than he does. People with star power, and who can put on an entire show, not look as if, as Simon put it (paraphrasing), "3 guys sitting around a campfire or dormroom". That is a major hurdle Kris will have to overcome.
Regardless if you like it or not, no one really does what Adam Lambert does, well, better than Adam Lambert does. He has a uniqueness that Kris doesn't. He will be able to pull off 2 hour long rock concerts, either by himself, or singing for Slash or Queen or Skull Mason, with no effort. And people won't be bored. There isn't anyone out there with a voice like Adam, especially for his genre (please show me if there is), while there are already plenty of people in the Kris mold who are better than him. It is weird to think about it, but, Kris is average at what he does.