I don't see why a song shouldn't go in an ad.
But I would add with selling out that quite often it happens to bands who have lost their creative flair anyway. I mean when a guy's in his teens and 20s he's generally got a lot of energy and passion but not so much of the skills to carry off all his ideas.
By the time he gets to his 30s the passion's going a bit, but there's enough there to mix with his superior skills. But generally guys in their 40s? I mean no disrespect to anyone here or anywhere but generally by that point they are experts at their instruments but I can't think of any band where I've really been able to say they produced their best work around 'midlife'.
I think by that point often the bands simply pick a trend and stick with it because they aren't the trend setters any more and what they had originally started as become dated. I don't know if it's really selling out or if they really believe that the music they've made is genuinely worth selling.