This answer is inadequate. They could easily put a feat in that does this at a cost. Something like taking damage each round, having to make a concentration check and if failed losing both at the end of each round, or your second concentration spell can only be level 2 or lower, or any number of things.
I have no doubt that something like this will creep in at some point since they seem to be throwing balance out the window.
Wait so either you have to track a separate pool of hp based on the monster you morphed it into, or this answer is pointless since 0 hp is dead. Either way its not a very satisfying answer.
Further I've already registered many complaints with the 'legendary/lair' mechanics: TL.DR: They basically feel like cheating. "I cast Hold Monster." DM "No, you don't, it suddenly moves out of range for no readily apparent reason not on its turn."