I love this thing. It's one of the most interesting and well built things I've seen in the workshop. But I don't see VALVe adding a mobile sentry gun. I'm not going to start an arguement about why it can't be added (it might be a little overpowered), but I would like to propose a different use for the model.
What if, the whole model was resized, and used as a replacement for the Tank in Mann Vs. Machine?! It would be a lot harder to have a Tank that actually attacks you. The normal tanks would still be used most of the time, but a huge tank with a gun on it would make a great boss or mini-boss! It probably wouldn't detonate it's own bomb though. It would have it's own track where it could travel in a loop shooting at the defenders while the other robots brought in their bomb.
I love the concept of a moble sentry, but it would be very hard to balance the stats enough so that the gun would be fair. And also, all buildings currently in the game aren't affected by gravity, meaning that if the ground under it is removed, it won't fall. So a moble sentry would have to start using gravity, and no offense to Gabe Newell, but I think VALVe doesn't want to go through all the trouble of rigging buildings to gravity over one mobile gun.
All in all, the idea of a moble sentry is great, the model and texture of the thing is great, and it is perfectly boned and set up. However, it would be very hard for VALVe to balance it out correctly. So I've simply come up with an alternative, so that this thing can still be added to the game.