I don't think you're wrong saying "It's Mitch's job to lose", if you mean there's a chance he won't start Game 1. I think Pace/Nagy would be thrilled if he's the best of the two. But look at it this way, they didn't give Mitch the 5th year option (yet, at least, and if they did, at this point, it would be a very luke warm gesture.
This story says insider says they declined it, but have not announced it.) That says a lot. Add to that Pace said there will open competition. What logical reason would you have doubt it?
Who wouldn't benefit from there being a real competition? Pace benefits if either QB does well. He got both QBs. He'd love it if Mitch does well, but he's not going to sink the Bears and himself if Foles dispays he's clearly better.