Sorry OP, have to slightly disagree.
Less about catering to all, more about abandoning your previous consumer base to pursue another one. Kind of like trailing off while talking to your girlfriend but staring at another chick. Big difference between that and talking to a group of your friends with your girlfriend included.
Game companies should feel free to try to draw in as many people as they want. I mean, they need to make money somehow, right? But they shouldn't just drop everything their current customers seek from their products. Sometimes, two different audiences will want things that are mutually exclusive. An example is that many on here before ME3 wanted and expected a fully interactive experience where we stayed in control of every facet of the narrative and story dominating over gameplay through a fleshed out single player experience. The crowd Bioware chose to pursue to widen their audience called for visceral gunplay over storytelling, non-interactive cinematic experiences and multiplayer. The two crowds don't really mesh, and the end product appeals to fewer in the end, relying upon past customer goodwill and interest to boost its own sales.
I, for one, only bought it because it was goddamn Mass Effect, not because any of the features marketed appealed to me. Rather, said features were almost enough for me to turn away from buying it, so much so that to my own amazement it wasn't a day one purchase like I had been saying for pretty much the past two years. It was only because I couldn't resist learning the outcome of my past experiences that I ended up getting it in due time.
In truth, I believe that without ME1 and 2, ME3 would have flopped pretty fast. To RPG players, its a messy experience with little player control . To the other side of its attempted market, the shooter crowd, its a weak combat simulator that relies too much on story and has no real meat to its MP to hold it up, with only a single MP mode and not much else going for it. But because we'd seen something great beforehand, and had been building up the hype ourselves for the past two years, drawing the interest of more than a few others.
In illustration, watch how well the Hobbit will do in the cinemas over the next couple of months. Doesn't matter about the quality of the film, lots of people are going to watch it. We could be looking at a decent challenger for top spot in movie opening weekends ever. We've experienced the universe before, be it through the books or the LotR films, and many are already foaming at the mouth to see it. What's more, in the years since the LotR films came out, more and more people have been watching, experiencing, building up the ecitement as they join the waiting masses. That's what ME3 had, albeit to a lesser extent. That's why opening sales aren't worth a damn.
Kinda slipped away from my original point there. I apologise. But the secondary point is just as valid!
TL:DR- ME3 didn't appeal to too many audiences, its just chose two that look for very different things, then handled the conflict between the two halves poorly. When it came to creative decisions, it chose the new market over the old one. Then things kinda blew up in Bioware's faces, and the downwards spiral has been pretty constant since.