I think that there is evidence of him having a loyal personality. Therefore, the chance of him staying and growing with us is higher than average.
You mentioned him wanting to manage Chelsea. Everyone will ask him that, and the only honest answer is: yes, one day. But, a great chance of managing Chelsea would come from a few years here and getting us some silverware. And if Derby goes on the up, may be he'll stick with us.
Personally, I think most managers ebb and flow in performance. But if we get a manager with the pedigree to grow, and a reputation for loyalty, then that will give us an advantage in the long term over other clubs who chop and change.