There were already plans for a third Mistral as a training ship, adding a fourth Mistral to replace older smaller ships is a good idea. One can never have enough sea lift. I would rather have the sea lift than having more ocean escorts, there isn't much of a submarine threat across the North Atlantic any more.
As long as any OPV, or LCS has the capability of carrying an ASW helicopter, ASW torpedo tubes, and a good sonar, you have a sufficient ocean escort. There are no need for expensive long or medium range SAMs.
Of course, it is a different story with carrier and amphibious task group escorts.