Personally, I think the exact opposite is happening: Mass Effect is building up steam. After all, the first game was (and please forgive the term) a MINOR major release, if that makes any sense. Yes, Mass Effect was a big thing, but compared to other major game releases of the time, it was overshadowed in the public eye a little, disregarding the Fox News scandal it had. Then it got a fanbase. In fact, if the fanbase had come along a little later, I would've been afraid that it would've went the way of Firefly, being cancelled simply because people took too long to realize that it was like a flawless gem hidden underneath a pile of rocks.
When Mass Effect 2 came out, yes, there were complaints about the changes, but I feel that is when the series really started to pick up (though don't get me wrong, I'm not saying its the better game, it just has more widespread appeal, as far as I know.).
Now, I will admit, I enjoyed Mass Effect 2 more, even though I truly miss the depth the world had in the first game. The galaxy truly felt vast and varied, where some races weren't almost completely one sided, unlike the second games Batarians. Yes, the inventory system was a hassle, and I definitely prefer the second games take on weapons and armor. Each weapon had a different feel, each had pros and cons, while the armor simply wasn't about better stats, but which bonuses suited your style of play best. I missed the Mako, simply because it was occasionally fun, though I feel that the Hammerhead as its spiritual successor, fills the old Makos role nicely. The mining minigame was something of an irritation, but in the end, having all of my squadmates survive? It was worth the work it took to get each and every upgrade.
To add to the wall of text, I am most definitely anticipating Mass Effect 3, and drooling over every tidbit Bioware gives us. I feel that they're continuing the experiments that they made in Mass Effect 2, further improving the combat, and adding more streamlined aspects of the first game, namely in the new weapon mods and the more diverse skill-leveling. Now, I am hoping that they show Talis face in the game (Yes, I am a hardcore Tali fan, but you can't exactly blame me), but I won't exactly complain if they don't, simply because I have one hope for ME:3. Please Bioware, PLEASE extend the romance scenes! They don't have to be overtly sexual, but give our squadmates, particularly the alien ones, more facetime! Pardon the pun.
Otherwise, I am extremely optimistic for the Mass Effect series, since I feel Mass Effect 3 could only improve on its predecessors. The only thing they could do better? Have them one-up The Elder Scrolls, and make an open-world Mass Effect RPG, where you can choose what race you want to be, your origins, and so on. I know they can do it, and while I don't want it to OVERTAKE The Elder Scrolls, it would make a nice, sci-fi contrast to it. And they know fans would jump at the chance to play as their favorite race.
tl;dr - I don't feel that Mass Effect is losing its touch at all. Its just getting better!