MP gamers have specific standards. If you stab them in the back or you don't deliver what they lay out for you then they can and WILL turn their back on you. Quite the opposite of SP. Bioware can make garbage and SP gamers will still pre order it and throw their money at them which is why SP will always be around and always be useful. That's not to say there aren't people in the MP community won't buy whatever the devs and commercials tell them to buy....there will always be people like that, but a large portion of it requires quality games.
It's a lot bigger than you or me and what we have played and what we prefer. It's the business....the gaming industry as a whole and what makes the most $$$$. Tell me....if you made a top 10 list of the most profitable games, how many on that list would be MP focused? Is it good business to maximize profits?
I play my MP just fine. Sure there are some loading screen issues and occasional lag here and there, but most of the time it runs perfectly fine. Perhaps SP gamers can foot the bill for our dedicated severs because I bet DA: I (which will be the last DA i ever buy btw) taught them nothing and the cash is already on the table.
All gamers have specific standards, and all of them are willing to let go of certain things if they enjoy the game enough, and MP-focused players are no different. Look at Destiny. Here you have a game that is often criticized by players left and right as being a huge disappointment and mediocre, even by players you might interact with within the game, yet people keep coming back and the community remains active. I've yet to have trouble ever finding a group for strikes or Crucible matches, and I've been playing since the day of release.
These games are super accessible and don't require a long attention span, and tend to stay alive because there are other players to keep you focused and compete with. Singleplayer campaigns that exceed the double digits in hours are always in a tighter niche, no matter how good the game is, because they often require more attention than most players are willing to give (which is why the rate of completion tends to be low).
SP gamers won't have to foot the bill for anything. MP players will do that themselves when they pay the subscription fee to have access to the game online.
I think the best thing about ME3MP, and I suppose Destiny, is that their respective communities are pretty nice, and don't seem to be at all hostile to newbs still getting their bearings with the gameplay.