All I can tell you is that from the original 15 Sterling MPL I know that 14 has found jobs in other airlines again.(last one stopped flying) This includes Viking(until they disappeared), Arik Air, Cimber-Sterling, Malaysian, Turkish, Jet Time, Primera, Norwegian and more.
Nothing I know of, limits (at least european) airlines usage of MPL pilots, as long as MPL is accepted as a license in their manuals just as well as CPL and ATPL has to be.
You are correct regarding the PICUS, but I believe that most respectable airlines does have such a program, correct me if I am wrong.
I agree with you in some regards when you say that MPL suites legacy carriers better, well at least the airlines needs a certain size in my opinion.