As for the snobbish elitism of gamers, I'm not convinced it's as common as you seem to think. Sure, there's always a vocal minority on the boards every time something like this comes up. But for a lot of people, a publisher's / developer's insistence on modifying a product for wider appeal signals a fundamental shift in design strategy and priorities. It shows a willingness to modify or discard other aspects of the game (which may be part of the reason current fans play at all), in order to lure other sorts of fans (which may or may not be interested anyway).
We have to understand that the gaming world isn't just divided into 'hardcore' and 'casual' as so many seem to think. Within either of those categories, there's a lot of diversity and variation in what we actually find enjoyable to play. Personally, I don't like first person games. It's not just the hyper-militarized power fantasies or shooting of all the foreigners (though certainly, that's why I don't play CoD/Battlefield), I actually find the perspective and motion unnatural and a little bit nauseating. I'll make the odd concession for a truly stellar game, but I'd rather 3rd person it up all day.
That's a small example, and there are many others. Point is, there are certain design choices in games that are popular, yet some gamers find unsavory. It's not elitism, it's a desire to continue to enjoy the franchises that you do in your little niche of gaming space, irrespective of whatever else is selling 11 million copies. And it's frustrating for many fans of an established series to see a pub/dev go this route, because those 11 million are already so catered to, while a gamer who doesn't necessarily enjoy what they do is left with a handful of titles per year. It's why people freaked out when From said Dark Souls II was going to be 'more accessible'. It's not elitism in the sense of 'I won't enjoy this game if you can' (at least not for me, and I suspect not for many others), it's a desire to see continuity with the design choices in the series that made people fans of the game in the first place - design choices that might have frustrated or alienated some other players.
Now, does multiplayer necessarily affect the design choices of single player? I'd argue (as below) that yes, it can.