Why was Optimus Prime killed off in the first place?

I mean Optimus is for all intents and purposes the "face" and lynchpin of the Transformers franchise. What I mean is that people who may only have a vague familiarity with the franchise, can none the less, still recognize him. Bumblebee is probably the second character closest terms of causal recognition.

I heard that they killed him off because they wanted to clear out the older characters as way or promoting the toys for the newer ones. So it really went down to merchandising and trying to freshen things up. Plus, and I think that Peter Cullen himself has said this, the producers likely underestimated Optimus' popularity, especially with children.