The thing about Crosby currently is that he's in a mental slump.
Bringing in another kicker isn't going to light a fire under Crosby because he's ALREADY thinking that he's lost his job (until McCarthy came out and said that Crosby will be our kicker). Bringing in someone would have just worsened Crosby's resolve and he probably would have just given up then and there. McCarthy is trying to instill that confidence, that swagger back into Crosby.
If your child fails a test you don't go to him and say you're going to start supporting his brother/sis more because of his failure. You sit him down on 1 knee and tell him that it's not the end of the world, that his skill is just too great and that he'll recover and be fine. As much as I hate our skill players being emotionally fragile, that is currently what Crosby is and in a way, McCarthy is right to be standing behind him the way he has.
However, being that the NFL is a business, if McCarthy has a viable replacement ready and rearing to go and believed in that kicker, Crosby could be gone tomorrow irrespective of what is said today.