I definitely understand why people have trouble letting go. I made this thread mostly because I was feeling a little bummed out that this was it, that I wasn't satisfied with the endings, and that I'll never get to hang out with Shepard's squad again.
Like many of you, I got more attached to the squad than to Shepard. I had several Shepards, and I played them all a little differently. Some of them I enjoyed more than others; I lost my save for my very first, and favorite, Shepard, so she never even got to see ME3.
The gist of it is, even though you can go back and play the games again, Shepard is gone, and may as well be dead. She or he might be dead, or not even born yet, in the next game. Knowing what happens--and I don't have MEHEM, but I also won't because doing so just, well... it's like denial or something--makes it sort of like watching videos of a deceased friend. Sweet, but painful.
I have absolutely nothing against people for missing Shepard or the ME series, but some people are a bit obsessive about it. Or depressed over it, which--again, I can understand--is kind of worrisome. Not in a "you must be nuts," way but rather, "you're getting very upset over this game" kind of way.
And I understand even more, because Bioware is the only developer to produce the kind of game Mass Effect or Dragon Age are. RPGs with intense depth of characterization, choices to be made that will shape future games, etc. I've heard good things about The Witcher series, but you can't port your saves in those, and having attempted to play the first one, they're not meant at all to cater to a female player. So no, there really isn't anything like Mass Effect, and until the new game comes out, there won't be again. Much as I say I won't buy it if it's a prequel or a side-story, by that point I may be missing the game so much that I will do so anyway.
Those of you who expect Shepard to make a triumphant return are most likely expecting too much. If the next installment of the series flops, that may just be the end for ME altogether. No more. It doesn't make money anymore, so why make more? If they did return to Commander Shepard, they'd have to canonize the Destroy ending. And you probably wouldn't be able to port your old saves.
What I do think might happen is, in another ten years or so, a remastered edition with all three games (not just the Trilogy Edition) will be released. It will contain all DLC content, maybe a few new side stories or missions, and perhaps even some new ending content. But if and when that happens, it will feel more like visiting an old friend--you will never, ever get back the first time you played when everything was fresh and new.
That's part of why I believe the story would have gotten stale; also, as someone else above pointed out, each extra game would have added more cast to keep track of. It would be odd to keep running into the same handful of people across the entire galaxy time after time. As many of us ladies as were disappointed that we lost several romantic interests as options in ME3, I expect had there been more ME games starring Shepard, those would not have been the only ones to suddenly become inaccessible.Male Shepard would have lost a few, too. While most of us don't play solely for the romance, I can almost bet that there would be plenty who'd quit playing solely for that reason.
So yeah, I would have enjoyed more time with my crew, but it definitely makes sense that Shepard's tale is over. Put him/her to rest, and enjoy the real world for a while, in which we don't have to fight a terrible war against the Reapers. You don't have to stop replaying the games, but, again, it's time to let go. There won't be more content. We may not even hear anything about the next game in the franchise for another year or two. Think of it as the Quiet/Quiet Citadel party. It's a little time to rest before something else begins.