Unreasonably long time spans.
Please please please don't ever let me hear "million years" again in any new continuity. It was never a good idea and makes almost every bit of storytelling way less credible.
Not enough likes in the world. Damn Costa's eyes for introducing it into the IDW-G1verse!
Though I would find it more acceptable if next time there's some lampshading that goes somewhere interesting. I'm imagining a human trying to wrap their brain around the age of the TF they're talking to. "Bumblebee, how is it even possible you're still naive and inexperienced when you're older than almost every species on this planet?" That could be a springboard to investigate the alien-ness of TF birth and aging and death. Like, is there a connection between the fact that they seem eternally stuck in their individual mental maturity levels and the fact that they're born adults and don't naturally die? How long did it take for "old" bots like Kup and Alpha Trion to get that way? The natural assumption is that it took millions of years, but what if it were much shorter, like even human-scale? And it's a thing that happens only rarely and kind of arbitrarily, disconnected from actual age - more like a disease than general entropy?
And of course right with that is the absurdity of them fighting the exact same war for so long. The contrast to human societies could perhaps be used to make a point about the value of death and rebirth.
Those are the best and worst of the fighters from both armies suddenly removed from the war. That's gonna change the dynamic of the fighting between who's left. Especially if we start letting there be multiple generations that come and go while the ships are off floating around in space. Technology advances, society changes, the factions shift, Optimus and Megatron are bland, sterile entries in history books. The returning warriors are entirely obsolete in every way - even Megatron who wants to exploit the new power (tech and political) is cast aside at every turn and Optimus must face the fact that he was playing part in holding Cybertron back because the advancements wouldn't have happened had he stayed.
Hm, like that one panel in G2 where Optimus is musing to Grimlock about what the Cybertronian Empire represents. I really liked that bit, and agree that it would be great to see it expanded. I mean, it's hilarious
that G1-toon Shockwave took his final promise to Megatron so literally. But the joke's been done now.