User blog comment:JakePT/Arrival has Arrived/@comment-1891865-20110330173914/@comment-1691295-20110331031726

Kenson says that if they destroy the Alpha Relay it will delay the Reapers for weeks or months, as they have to travel to another relay.

That explains why the Reapers don't show up, but it doesn't explain why they're so far away in the final cutscene or why no one seems to remember or care about what just happened.