well it will probably end with the single most BAMF death they could possibly think of for shepard, which will then cause questions of not only if humanity's role will be beneficial to the galaxy at large's long-term survival, but also if humanity will survive in the long-term at all. because there will always be more reapers, and with humanity's only hope dead for good, there wouldn't be anyone obvious to stop them.
Proud of the Six One Seven.
We are Boston. We are strong. We will prevail.