I understand that often the MPL is described as a licence which ties you to that airline as a First Officer with no possibilities to change airline or become a Captain. That quite simply is not true (from my research) - after 1500 hours an MPL will convert to an unfrozen ATPL which makes you just as qualified as those who follow the white tail route. I am willing to stand corrected on this, but I am pretty confident this is the case.
Nail on the head. Anyone who thinks in 10-15 years time an airline's chief pilot is gonna say "this guy is just as qualified as us all with his 9000hrs and ATPL, but he can't be a captain for us ... he did an MPL" is not really with it if I'm to be rudely honest.