Why MIB is "stuck this way" by Ross Miller

Apologies if this theory has been put forth before by anyone else but I just wanted to post it here to get your thoughts. Here goes:

When Ilana, Sun, Ben and Frank are carrying Locke's dead body to be buried with everyone else who has died on the beach, Ben asks why they had to bring Locke's body. She replies that they needed to know what they're up against. And Ben then asks what's to stop what they're up against from changing into someone else, Illana replies that he can't because he's stuck this way.

Now why would that suddenly be the case? My theory is that it's as a result of MIB finding his loophole to kill Jacob. Jacob made MIB the way he is i.e. being able to change into the Smoke Monster and manifest as other people, and MIB's ability to do so was always tied to Jacob from then on (remember MIB said to Sawyer in "The Substitute" that he used to be a man like him). This would also be the case with Richard i.e. Jacob made Richard that way (Richard said so himself near the end of season 5). But now that Jacob is dead, MIB's ability to change into whoever he wants whenever he wants has been taken away and he's stuck only being able to change between the forms of the Smoke Monster and the last person he manifested as who is Locke.

How this affects Richard being ageless remains to be seen, but my guess is he'll just start ageing normally (he won't, for example, suddenly age really quickly into a really old man or crumble away) now.

Anyway, I know there are probably holes in this theory (isn't there always :P) but I would love to hear your guys' thoughts on it.