If Morgan Stanley are willing to stake their credibility on an assumption I'm going to take that assumption seriously.

This is pretty silly thing to say. Investment banks suffer zero reputational damage when their analysts get this sort of thing wrong. They don’t even have to care about accuracy because there will never be a way to even check this number, if anyone even wanted to go back and rate their assumptions, which also never happens.

Calling this unmotivated assumption an "estimate" is just plain lying though, regardless of the faith uou have in the source of the assumption.