What if it is something like this. He is brought back from (near) death, on the condition that his life can never be saved in a similarly magical way again. That is, whatever intervention is being done to interrupt death can only be done once to any person. If it were the only option to save his life, of course they would accept that price, because if he died then and there, there wouldn’t be a second chance to save him anyway.
Matt had suggested something similar. I was thinking about it earlier today, actually, and pondered this: if this theory is true, then why was Rumple at the end of 3B so determined to learn how Nealfire died (in Emma’s present day) so he could save him?
I have no doubt that the theory is really solid and seems like an explanation we’re likely to get, but it would feel slightly retconish.