Yes, that's one more thread about the ME3 ending. If you don't care about it, don't read it. Also, heavy spoilers here, of course.
I've been thinking about the endings since finishing the game last week, and my conclusion is that it was a good idea with clunky execution. I don't love it, but I also don't hate it. But this is not the issue here.
Many people say that things are "back to the stone age" or such if the relays are destroyed, and that the fleets are stranded on Earth because of it. Every time I read this I thought "don't people remember the ships have Faster-Than-Light travel capability?" and then I wondered if it would be feasible to use FTL to get to distant places in the galaxy, without the relays.
(It's clear that the deactivation/destruction of the relays will NOT necessarily happen as in the Arrival DLC. The Alpha relay was destroyed externally, by an asteroid crashing into it. The relays in the ending of ME3 are deactivated by their creator, so it's quite possible they were intended to have a deactivation procedure that didn't kill everyone near it.)
It turns out that it is, indeed, quite feasible to travel the galaxy with FTL only, even if much slower than what was possible using the relays. The estimated diameter of the milky-way is around 120 thousand light years (source: Wikipedia [https://en.wikipedia.org/wiki/Milky_Way]). One important thing was knowing how many times over lightspeed could the ships travel. According to the Mass Effect wiki [http://masseffect.wikia.com/wiki/FTL] (apparently citing from the Codex) a starship with a mass effect drive can travel around 12 light-years in a day of travel.
This means that to go from one end of the galaxy to the other, starting from an edge, it would take around 10 thousand days or 27 years with the FTL drives in current ships in the ME universe. A long time, for sure, but well enough inside a human's life span (which the codex mentions that can last around 150 years at the time). And this is the time to cross the whole galaxy in a diameter, with nearer places being much quicker to reach. So it's completely possible to establish routes and get to many planets of interest.
Considering people have much more knowledge about reaper knowledge after the end of the game, it is even reasonable to assume they can eventually replicate the mass relays.
So there, the prospects are not so grim