I think the word sell-out is when you take a decision which you would be against in normal times, but you have to take due to economical pressure.
Bands that become more commercial are sellouts? Well, if they have choosen to make different music only because that's an artistic choice, they aren't. If they write something just so that it becomes a number 1 hit, yes, they are sell outs, and actually, they don't deserve to be artists...