To be honest mate, i think for once you are missing the point of why we hired Koeman.
I don't believe for one second he hired him to be a manager for years to come and win the lot. I think he is limited somewhat as a manager as well, again in the moyes style of limited, not going to take a group of mid table players, splash the odd bit of talent and expect to beat the money clubs.
However, i believe we hired Koeman for very basic reasons. His job is to come in, make the team solid again and hard to beat and get us into europe again. If he does that then job done as far as he is concerned, the better he does then better for him.
you say he is using us as a stepping stone? He tends not to stay for more than 2-3 years at a club anyway, i would fully expect him to go in 2-3 years, all evidence points to that. But if he gets us back into europa at very least whilst he is here then the club has a much better standing than before koeman arrived.