There's a positive and a negative to retaining him.
Positive: Was there a better option out there to replace him with now? Most likely not, but maybe Ozzie Newsome looks to get back into a GM position after next year after taking a reduced role to stay in BAL.
Negative: If ARZ finishes below .500 next year, then Keim has to go but the problem is that there will be a new coaching regime next year and if Keim is axed then the staff has to go too and that means a fourth OC for Rosen and yet another new system to learn.
I think Keim goes after McCarthy as he is probably the most established and decorated offensive coach on the market this year. It will be interesting what the new DC does, stay at 4-3 or return to 3-4? Whatever the scheme, Keim needs to get the right players to fit the new scheme instead of hoping guys can transition into those roles. He has plenty of cap room to bring in guys, hopefully he is not looking for basement bargain guys who come cheap either because they are always injured or over the hill. The holes on the team are obvious, (LB's, OL, WR & #2 CB), not sufficiently addressing these areas is going to seal his fate next year.
One thing is for sure, if we don't finish above .500 next year because of a busted draft and FA, Keim has to go, no more chances. Especially if he hires a proven coach and an upgraded coaching staff.