Could the Collectors & Reapers have been defeated... without Shepard?
Let's say Shepard was never revived in Mass Effect 2, and instead Cerberus contacted one of Shepard's former companions to stop the Collectors. If that mission was successful, they would lead the charge in the Reaper War. Think of it as a Darkspawn Chronicles "what if?" scenario from Dragon Age Origins. In that DLC we know how far Alistair got without the Warden leading the team, and we also know that he loses the Blight in the end confirming that the Warden was the difference between victory and defeat.
Do you think it would be the same thing concerning Mass Effect? If Garrus, Ashley, Kaidan, Tali or Liara were contacted in ME2 (and they accepted working with Cerberus) do you think they would've been successful? Or what if the Illusive Man put Miranda in charge of recruiting a team and stopping the Collectors, do you think she could've done it?
And lets pretend that the person was successful, do you think they could've lead the Milky Way to victory against the Reapers?