I think those were just the last intact protoforms. I could see Magnus maybe using one as a quick repair. But 1) I'd assume there's other protoforms on Cybertron. 2) Prowl's spark is gone, so there's nothing to transplant. 3) Same with Blurr. The image we've seen is just a model of a concept that it is freely admitted never made it into the show, which means it can be taken as canonical. Blackout's earth form ain't in either. Not to mention: NO ONE KNOWS HE'S DEAD! Hell, no one has a reason to suspect he's dead! As far as everyone's concerned, he's off somewhere with Thundercracker and Skywarp. Not to mention, Cliffjumper threw him in a waste disposal. Good chance his cube body got destroyed.