See this is where we get hung up again on esoteric religious terminology for mundane things. You just basically said "I've been ecstatically happy at times, but if I was constantly in that state, it would suck." Why do you need to paste on terms like "Heaven Realm" to that?
No one is arguing with your point. in fact, the second video I linked addresses that very point. A life without end, a delicious cake that can never be finished, would quickly cease to be a pleasure.
But there is no such thing. As you said, you cannot remain in the "Heaven Realm." It is literally, physiologically impossible for a nomal person to just remain constantly ecstatically happy.
The "Heaven Realm" does not exist, because it's just a fancy way of saying "a state of happiness," but adding on a caveat against an already impossible condition of permanent happiness. Which is just plainly absurd, and seems, again, to be best explained by fearful people reacting to the question of how suffering can exist in a just universe by refusing to admit that the universe is simply not just, and instead deciding that suffering is somehow virtuous by way of cautioning against a permanent lack of suffering (again, a state that has never existed.) It is archaic and primitive rationalization in the face of a terrifying universe, instead of a mature coming-to-terms with the true nature of things.