Well the EC made it so the Relays were not totally destroyed, though it still shows the Charon relay totally fucked up and in pieces regardless of your preparedness. The perfect Destroy ending only has Hackett saying they were severely damaged. If your preparedness was under 1900 he says it could take years to repair them. This is probably the most crucial piece of information in all of the ending. How they choose to deal with this will have profound effects on how things develop in the Galaxy.
Even if BioWare makes it so that they can repair them rather easily, there are two major issues still at play. One you need two working relays for them to be functional. Even if you fix your relay, unless their is another working relay on the other end you're shit out off luck and have a very large and useless satellite you spend years fixing. Two most of the galaxy was nearly wiped out. Most of the galaxy put everything they had into a last stand in Sol. So it wouldn't be very unlikely that most of the galaxy would be ill equipped to repair a Mass Relay, let alone have the knowledge to do so.
Taking all that into consideration even if you did have the know how and means to repair a relay, which I doubt most systems would. You would still need to spends months if not years traveling by FTL to reach it's partner relay in order to repair that so the system can work.
In all this would likely result in a patchwork of working relay systems that would take years if not decades to fully restore. Even then again more distant and remote region would likely be cut off for even longer
I would love to see how such a situation would evolve. I mean you have the simple stuff like a human or Turian system or colony that was cut off for decades and developing into it's own little society and culture. But then you have the possibility of large portions of the Galaxy repairing a handful of relays into their own little network after a few years and then spending decades alone before they reconnect with other large networks. How would these various regions and governing bodies react when they reconnect with other areas of the galaxy. Would everyone be happy to bring back the Citadel. Or would they rather maintain the governments they have in place.
It's things like this that make me very wary of how ME4 will be handled and the rather high chance BioWare will just act as if nothing happened and return the Galaxy to normal. Because after an event like the Reapers nearly exterminating all life in the Galaxy, things just don't return to normal. You can't just turn the clock ahead a little bit and act as if nothing happened.
One big factor BioWare can definitely play around with though is all the dead Reapers everywhere and the technology/knowledge they can extract from them. The Protheans, on their own, eventually figured out how to build a relay small enough to fit on a large ship. Who knows if the races stuck in Sol can't just figure out relay-speed travel for individual ships with everything they salvage from the Reapers? Not to mention all the secret shit in the Citadel the council spent 2000 years ignoring.