What gets me is that, having seen Cullen's in-person thoughts about the movie, it's not something he was very fond of, since the movie didn't just kill off his character so early into the runtime, but it was basically like his pink slip from the series. The movie essentially fired him from what was one of his favorite roles (if not his most favorite role) of the time (plus Ironhide, who also got taken out even sooner), and he even wondered back then if maybe it was his own fault, as if he had unknowingly done something wrong that got him dismissed, before learning it was simply a stoic business decision to sell new toys by getting rid of the old ones.
In several convention panels and interviews, he recalled reading the script for the first time, being taken aback by Optimus's death scene, and then getting to the last page and being like, "I... I don't come back...?" He remembered turning to Frank Welker reading the script next to him and saying something like, "Hey, Frank, have you gotten to the end yet? I'm getting whacked!"
And not only was Optimus a favorite role of his, but it was also his most steady role at the time in terms of income. He relied on the G1 cartoon in order to help him stay financially stable. Then the movie came along and he was like, "Well, great. There goes that paycheck!"
Knowing all this, I am so glad that he was able to bounce back with voicing his second most iconic role: Eeyore (up until Disney took that away from him, too, a couple decades later, but I digress!).
TFTM may have been one of his most remembered works, but it really did him dirty and paid him very little respect. That's show business!