Few things to keep in mind.
1. We've come to find out, more or less, that not all reapers are the BIG, capital ships Sovereign was. Granted, there are prolly alot of them big ships, but in general, it's not a fleet of "Sovereigns" like it's been made out to be.
2. Sovereign was taken down, more or less, but one wing, or fleet of earth ships, not the whole earth navy, and what was left of a suprise attack on the citidal that started with them knocking out communations so they couldn't call for help. There were alot more reinforcments to be had, if they'd known they were under attack in the first place.
3. The reapers are burning ALOT of resorces and fuel to make the trip to the cidital. Not only did they have to come back from dark space, but once the alpha relay was blown, that added possible up to a years worth more FTL travel, which has to be taxing.
So i think there's a SLIGHT misconception as to how powerful the reaper fleet really is, IE, they aren't all Sovereigns, and are buring ALOT of resorces right now, and we aren't as WEAK as we seemed. It was a suprise attack, and a fraction of the galaxies full milatery might. A head on fight, with the WHOLE galaxie, as in, EVERYONE, might be more then the reapers can handle.
After all, all reapers invasions START with a sneak attack that cripples interstaller travel. where they can pick apart picemeal resistince one system at a time. That won't be the case this time. It's going to be a head on fight, after a long exidos, with a unitifed galaxie (once i'm done with them). Something they may well have NEVER faced before.
And of course, there may be a magic Maguffin. That can always happen to if the writers get lazy... >.>