I am not a big expert on this, however I would say that it is not more of a detterant for the same reason that penalties for stealing aren’t more of a deterrant, and that the clear statistics about how abortion messes up women isn’t more of a deterrant. Firstly, many don’t realize these things. Mostly, however, is concupiscience and the nature of sin. Sin is in and of itself a rejection of what is known to be good for what is known to be bad. When a person is going to depart from good to choose bad, they are already making an irrational decision, a decision in which they think they know better in some way. It’s not a detterant because people are by the nature of sin shunning the better in favor of the worse.
There are consequences to every action. The church says no to optional celibacy. So what happens if a priest leaves because he cannot accept that? What are the spiritual consequences to that decision? Any or none? I know that there are many practical negative consequences and that alone should be a deterrant, but it doesn’t appear to be.
Any priest that is indeed, a priest, knew those things that were being asked of him at the time of his ordination. Priestly formation is a long process for a reason. With that being said, if the Priest joined in anticipation of the Church somehow changing Her mind about madatory celibacy then he joined for the wrong reasons, anyways. Priests take vows and these vows are to both Church and directly to God and as such, a Priest must ask forgiveness of both the Church and God for not living up to the vows in which he had made.