True, which is why he won't kill him off, but I could definitely see him in a position where it's a cliffhanger if he's going to live or die.
But yeah, Optimus won't die, at least not in this one. Fandom is too attached to him, and hopefully Hasbro learned from the last time they tried to off him. Fan reaction to the early leaks on the first film got a nice bump when Cullen was attached, so I'd say it'd be suicide for them to mess with that.
If the Constructicons are going to be in the film, they're going to need some way to top it and hook people on another film. Putting his status in the air, only to save him in the 3rd in time for him to make the ultimate sacrifice would be one way to do so.
IF they go the Unicron route or something for the 3rd, chances are, Autobots and Decepticons are going to have to work together and Prime will be the key to everything. If the treatment for TF2 was true, I could see said Decepticon begrudgingly saving Optimus if confronted with such a threat.
Not to mention the whole idea of "the fallen" (if not in reference to THE FALLEN) could establish some sort of mortality/resurrection status quo which would play into a third film.
Just speculation really.. I know nothing
EDIT: Imagine if they leaked an image of Optimus (2009) ala 1986..
Imagine all the talk that'd generate