"Hire" all the humans they needed? They wanted resources and a workforce to help rebuild Cybertron. There are plenty of workers here on Earth, granted resources are becoming questionable. There was an alternative to the whole go crazy and kill everyone scheme. If you look at the world today there are hoards of people unemployed or living in underdeveloped areas that would kill to have an opportunity as awesome as going into space, living on an alien planet with what amounts to a totally secure construction job for the rest of their working days, and probably their next few generations of offspring. You simply take the Space Bridge set it up in the same orbit as the asteroid belt, use all the Cybertronians that are left to mine the asteroids for whatever resources they need. Bridge it to Cybertron, the Humans then do the building and away they go. I would wager it would be far easier for the Cybertronians to do the mining than the humans. Think about it, the best mutually beneficial solution one could come up with! Humans get jobs and for some probably better living conditions, A chance at new lives basically, the Cybertronians get their planet back, heck while they are at it, they could work on "augmenting" Earth a touch with some Cybertronian tech, new space station, moon base so on. I realize it would have taken longer than their scheme, but that was basically doomed to failure anyway. There would of course be some details that would need to be worked out, like I doubt Cybertron ever had an atmosphere so you would have to have short term habitation modules with a livable ecosystem for humans on Cybertron, but that would just be a part of the preliminary phases of the plan. eventually leading to a domed city with massive hydroponics and such, hell we can do that now anyway. Would seem to have been a scheme Far less likely to fail due to the whole pissing off Optimus hence forth getting spines and heads forcibly removed component of their original plan.