Okay, I see what you're all saying. What I'm saying is that the reach of the whichever ending was selected might not effect a distant galaxy the same way that it did the Milky Way. They might be using ancient, dormant Mass Relays this time and jumpstarting them, or they might have just managed to recreate a Mass Relay for the first time in ever, after centuries of trying. It would still be technology taken from billion+ year old space Cthulu.
The explanation that the story provides might be perfectly reasonable in context. Of course it's not going to be hard science or hold water against careful scrutiny, but that's been the way of this technology from the beginning. It's always been space magic, so that's probably what we should expect this time around as well.
I've been assuming that traces of inactive Reaper tech would linger, even if they themselves are destroyed.
i think the main issue is that Mass Effect lore started strong with the first game, started to crumble with 2 to streamline it into an action game, then by 3 it was a whithering husk full of (plot) holes that made no logical sense, and this upcoming game looks to be even worse in that regard.
No. Would you be surprised if I said I was religious myself? 
I'm saying a religion based on Shepard is stupid. It shows me these people aren't engaged in critical thought. There's little basis for it. Except maybe one case, when you ally Geth and Quarians (and maybe choose Synthesis later). Garrus himself calls you saint-like after that.
Low EMS Destroy is screwed... and the Normandy itself is stranded. If anyone is alive at all. Humans probably wouldn't be around much for sure.
The idea is Shepard became legendary....but it doesn't make sense because this is a time where there would be audio/video records of Shepard. So he wouldn't have become THAT type of legendary, unless all of galactic civilization was reset to the stone age following ME3, losing all records and technology, which just raises even MORE questions of how they became space worthy again, in the exact same manner, minus the tech they used to do it the first time.
and yeah, most humans were wiped out, and we don't have the advantages of Asari or Krogan to make up for it. Not just Earth's population, but between harvesting in 2, and obviously they would be targeting human colonies as a priority throughout 3.
Someone who calls a mere man or a woman "The Shepard" IS stupid. Especially in my case.. when Shepard was basically just a soldier, with really bad social skills. And much worse. They weren't listening to anything about me at all.
If he's indicative of their culture, then they're probably failing at having any institutions that champion critical thinking on these remarkable levels you wish to give to them so quickly.
As for the rest, that's how civilization develops. People rely on competitive behavior and hunting at their most basic. Then move on to agriculture and building communities (and in turn, housing said communities..which takes construction and developing the necessary stone, wood, and metal working skills along the way). And so and so on forth. This isn't headcanon. It's just anthropology. You can't jump to godlike levels when you start off with basically nothing.
I'm totally willing to hear things out.. It's why I'm talking to you. But I can't control how my mind brainstorms how these situations play out. To quote Lewis Black... "..... I have 'thoughts'". 
When I take the worst case scenario into account --- and then bring them to the point where they look like people in that new trailer -- then I have to guess it's been at least thousands of years since ME3. Not hundreds for sure.
I suppose....one possible handwave explanation is that the future civilizations drew inspiration from the remnants of Council Space galactic society, much in the same way they drew from (what they thought were) Prothean ruins. As in, they DID start from the ground up, but leapfrogged after finding ancient tech, only in this case, the "ancient tech" is that of the original trilogy, rather than prothean/reaper stuff like before. This would kinda explain why there's a random N7 symbol (They don't understand the meaning), and why everything generally looks familiar, despite the supposed time gap.
seriously though why/how is there a Carnifax?