There's nothing inherently wrong with doing something just for the sake of the majority. Say you're throwing a party with all your friends over and even a lot of people you don't know and you want them all to have a good time. You're next to the stereo trying to decide which music to play. Is it selling-out to put on music that the majority will like? No, you're just trying to make people happy.
You need to define what selling out really means to you, otherwise you may continue to have an unclear idea about it and hold yourself back. I think your definition of "selling out" is doing something for money that violates your principles. Figure out what your principles are, what you want to accomplish with your music, and then just stick to them. For instance, say we're back at the party. If you're someone who likes to promote the equal rights of women, then playing gangsta rap because that's what everyone likes would probably be "selling out." But that's different from just playing what the majority likes, and this is what you need to figure out for yourself.
If I had to make some WW2 shooter game that was originally designed just to appeal to the masses, I would give it all the mass-market appeal required by my employer while at the same time inserting my personality and influence to make sure it coincided with my principles. For example, one of your squad-mates would always wonder why the Germans were doing this, and he would refuse to swallow the propagandized answers the others gave him. It would be a minor element, but it would remind the player that the Germans must have reasons for what they're doing - that "because they're nazis" isn't a thorough investigation, and is actually dehumanizing.
The only time I would really refuse to do something was if I was not allowed to explore my own themes like that on a significant level. Then the game would be a waste of time for me, and I can't bear the idea of wasting an entire development cycle's worth of my career and my short life on this planet.
Lastly, the music you become known for is probably the music you'll be hired for. A company isn't gonna hear you make a sweet techno song and then hire you to make country. If all the music you promote yourself with is what you like to make, then the music you'll be hired to make will be too. (At least, that's what it seems like to me. Someone correct me if that's not how it works)