There is no way to know the answer to that question.
First, we do not know for sure what the Cardinals' plans are for naming a head coach. Second, we do not know whether McCoy will become a better head coach than the person Arizona winds up hiring in the end.
We do know Cardinals president Michael Bidwill announced intentions to interview Andy Reid, only to have Reid accept the Kansas City Chiefs' offer without visiting Arizona. We know reports suggested the Cardinals sought a second interview with McCoy, only to have McCoy cancel that interview after accepting the Chargers' offer.
So, from outside appearances, the Cardinals appear to be struggling in their search for Ken Whisenhunt's replacement. They do have an insurance policy in defensive coordinator Ray Horton. Missing out on Reid and McCoy would hurt more if Horton also appeared likely to land a head coaching job elsewhere. Horton appears more likely to stay, however.
In my view, firing Whisenhunt made sense if the Cardinals were in position to move decisively for a superior candidate. They have not done that to this point. Still, it's tough to render a verdict on the process before the Cardinals have made a hire. And even when they do make a hire, we won't immediately know whether they've made a good one.
Before hiring Whisenhunt in 2007, the Cardinals reportedly conducted second interviews with a group featuring Mike Sherman, Norm Chow, Cam Cameron and Ron Rivera. Whisenhunt was also a candidate to coach the Pittsburgh Steelers at that time.
Horton, Steelers offensive coordinator Todd Haley and Cincinnati Bengals offensive coordinator Jay Gruden have interviewed with the Cardinals this offseason. They appear to be the leading candidates.
No coach is going to succeed in Arizona without upgrading the quarterback situation. Whisenhunt proved he could win with a top quarterback. He lost his job because the quarterbacks he helped acquire and develop following Kurt Warner's retirement either could not stay healthy (Kevin Kolb) or struggled (John Skelton, Ryan Lindley) or both (Kolb, to varying degrees).
The other teams seeking head coaches generally have superior quarterback situations. That makes those teams more attractive to coaching candidates. If the Cardinals wind up settling for a lesser candidate, then they would have been better off keeping Whisenhunt, shuffling the offensive staff and making another run at finding the team's next quarterback.