It goes without saying the tactics, the players, the fitness etc etc all have to be taken as read. But that "It's the tactics, stupid" argument assumes that application of the tactics is always what it should/could be, and that coaches always keep their heads and don't apply incorrect tactical gambles.
It's not rocket science, for example, to maintain defensive cover against a team that can score goals, and wont give them up easily. To remain patient, and disciplined, til superior talent wins out, as it almost always does. The real question here is that when that approach breaks down, why does it happen? Why do otherwise great coaches, and great players sometimes make much more risky or even careless decisions?
Because everyone can be prone to major error, given the right set of circumstances. This is true in any walk of life. So what we are really debating is, what set of circumstances?