A&C and Betpump5,
The requirements for an MPL to switch airlines, both before and after 1500 hours are identical to those required for a CPL/IR holder. If moving to the same type, they need the new airline's conversion course to learn the SOPs. If moving to a different type, they need a new type rating. The only problem the MPL holders will have is the widespread ignorance of the above, but hopefully the airline recruiters are better informed.
There are many people skeptical about MPL, many who wouldn't hire them if they were the recruiters, etc. But the arguments behind these opinions have nothing to do with the regulations.
Your arguments on the regulatory side make about as much sense as if to say that a circa 9/11 Aer Lingus, BA or BMI cadet couldn't move on to a different airline because they were trained for a specific company. Plenty of them were dropped by those companies immediately after 9/11 and went on to get other jobs.
I think the debate about the merits of MPL are healthy and needful. Wannabes considering undertaking it will benefit from reading a large variety if viewpoints. They need everyone's opinions to properly assess the risk, but they won't benefit from muddling of the regulations and training requirements.