David always sounds like Peter Pan to me, very immature, a grownup with a teen mentality. If I was in his position, being a no more than average musician and having a genius big brother, I’d certainly embrace it rather than helplessly fighting for room in the band.
He had the great courage to leave a highly succesful band to do his own stuff rather than fulfilling his others' dreams.
A path the Mark will ultimately follow... 12 years after his brother! Just provoking: which brother is most immature? 
It makes no sense, if DS was Mark’s band not David’s how come only now he followed his dreams? And I’m not saying David’s immature for leaving DS, he was allowed to, he was young. I say that because he’s old now and keeps bringing it up all the time. Get over it, man.