Stoutflies, I realize I'm probably taking your comments more personally than I should, and probably more personally than many others here, so I'm attempting to rebut them only as far as the comic goes (because personally I get the feeling you don't feel like art is a valid profession, which may not be true, but the tone of your comments strongly suggests it). You sound as if Jeph would have to write the characters and plot in a way that would seem inconsistent with their established personalities. So far in the band you have Marten, whose only real passion seems to be music, Amir, who would probably just tag along for the ride, working only as hard as he had to, but be pretty excited about being able to do it, and Hannelore, who loves to challenge herself, and is generally good at whatever she puts her mind to (and an excellent drummer, from the way she talked about it when she started). Then you have close ties with Sven, who plays the local circuit, as well as writing songs for the mainstream. None of the characters in the band so far have any ties that would prevent them from touring, as far as I know.
There will require some character development yes, but I only see that having to come from Marten, and given his age and status, it's about time for him to wake up and put some effort into following his dreams. This would be just what I said though, character development, and we would be left with the same Marten, just more mature and more driven. This isn't inconsistent unless you want the comic to be one of those where the characters never age or develop, like the Simpsons or something. I don't think you're looking for that though, since you suggest school as a "proper" way to develop Marten. This would leave him possibly more financially stable, but would do nothing to improve his quality of life.
I'm in the same place as lepetitfromage right now, being a long-time musician who tried to make it, then life moved me in other directions, and now I'm horribly unsatisfied with my current status, despite unbelievably lucky circumstances. Often it seems that the only time I'm truly satisfied is when I'm writing, rehearsing, and performing my music in my spare time. Once I'm free to do so, I plan to pursue my music career again with full vigor.
I guess I just get upset when I hear/read something that sounds an awful lot like, "Your dream is too hard. Give up on it now and save yourself some time." Apparently this extends to when people say it to fictional characters
Truly though, I harbor no ill will toward you. I just feel like I have to say my piece. I'll do my best to drop it, or at least minimize it in the future.