However I would wager that over the next few years more pilots will enter the pool than leave, primarily due to the airlines that once recruited from the hold pool running their own pre selected schemes.
Clearly that is a trend which could work against the generic integrated route however if current trends continue, even with moderate expansion the current MPL numbers will not cover requirements.
Now MPL recruitment could of course be increased in the future (which would in theory make it easier to get onto MPL...) but I doubt
in the good times it will ever be a complete solution for the airlines because it involves planning further ahead than they often want to. Also it is not unheard of in OAA's case to take people from the integrated course and internally put them on an MPL course if they are still at the ground phase. This was done with the last easy MPL.
Edit to add:
I have just seen john's answer, thank you for that...I have taken on board what you say along with all the other opinions I have heard.