The license is good ONLY on 1 type and for 1 airline. There is NO valid upgrade path to ATP and Captain (other than paying for their own on the side, which they likely cannot afford) of which I am aware
Absolute nonsense. MPL is competency based training leading directly to the function of an airline first officer. The upgrade path to both an ATP(L) and subsequent captaincy is exactly the same as for any other path. The licence upgrade (which is unrestricted) to an ATPL is performed once the applicant has the requisite (1500+) hours and (normally) completes the few extra specific requirements as a part of one of their regular airline competency checks.
You need to do a little more research.