Dude, TF:TM rocked my boat, back in 1986. A scarring experience, and yet I still love it, as well as much of the third season. You want to talk “subverting expectations” (and, “let’s kill everyone so we can sell new toys”), then there ya go.
To this day, I’m convinced that the TRANSFORMERS franchise would not have had such longevity without that huge line being drawn in a sand. Sometimes a legend needs a definitive and unforgettable ending to make it immortal. YMMV.
Of course, Optimus Prime also got the kind of epic, heroic death that James T. Kirk was cheated out of. In the case of the latter, I would have preferred the pre-GENERATIONS approach of never mentioning or seeing what became of Kirk, and letting fans speculate.