First of all, how would Silas know anything about Megatron's project if he's been contained all this time?
How would Silas know anything about Megatron's plots? Daaah, gee - he's been alive and on the ship for quite a while. Ya think he might... ya know... HEAR things? It wouldn't be the first time that a character on this TV show improbably relayed information that Vehicons, Insecticons and even Megatron were casually discussing while strolling the corridors, or shooting the breeze in their labs. Weren't Knockout and Starscream doing just that - discussing Project Predacon, and other things, RIGHT THERE while Silas was twitching on his slab? 'Daaah, how would he know', indeed. He'd know a lot more than Airachnid, who was in stasis the whole time, and who knows absolutely nothing about any of it.
Secondly, Knock Out and Airachnid are possibilities as well, especially since both have inside knowledge and would be more useful to the Autobots.
Uh - yes. I believe I already listed Knockout as a possibility. Thank you for agreeing with me.
Could it be Airachnid? Maybe. But Soundwave dumped her and all the insecticons onto one of Cybertron's moons with no way to escape. So I find it much LESS likely that it would be Airachnid than Silas, who is still on earth, and by no means 'certainly' dead.
That final "thank you" of his was the key, indicating that he saw her killing as a merciful, freeing him from his suffering to let him rest in peace.
Saying 'thank you' while Breakdown's body makes a kind of power-down humming noise is hardly a certified death certificate, especially in the world of TV-dom. Silas may not have to depend on Breakdown's body being functional in order to stay alive. Besides, he has bits of Dark Energon in him. Ya know - 'bring back the dead' and all that...? Huh? HUH?
With only five episodes left to cover their tracks and little time to open up new story opportunities, I sincerely doubt Silas isn't dead.
If 'gee they don't have many episodes left' is your sticking point, that's all the more reason to use Silas, an established character with a pre-built model. It'll be a lot easier for the writers to have Silas recover, be captured, and interrogated as opposed to writing a way for Airachnid to travel light years away without a spacebridge, without rocket propulsion, and without anyone on Earth who wants to bring her back.
Thirdly, if they play the Starscream card again, it would just seem repetitive and had gotten old by then.
Gee - repetition on a TV show. Yeah, I've never heard of any TV program doing anything like THAT before... /sarc
Finally, are you suggesting Breakdown return to full life? If not, how would Breakdown's corpse regain health? If so, the writers strictly prohibit any character from returning from the dead. Prime treats mortality different. If you're foolish enough to die, you stay dead.
Daaaah, gee - Dark Energon? Daaah, gee - re-animates the dead? Daaah, he died before when Airachnid killed him, yet he's STILL in the series? Doesn't sound like his body 'stayed dead', now does it? Obviously the writers don't 'strictly prohibit' meddling with the limitations of mortality now, do they?
Not sure why Airachnid-lovers are so dang insistent that SHE be the one, and fight to try and dismiss the notion that it could be Silas, who has just as plausible a chance of being the 'captured enemy' of 'Minus One' as anybody. However, in the interest of fairness, I will throw the Silas-haters a bone and add this caveat: We're all just speculating. Until they show 'Minus One', it's all merely educated guesswork.