How about, what isn't a valid contribution to society? What does a contribution need to do in order to be "acceptable"? I mean, I'm not killing anyone, I could argue that by not killing anyone I am contributing to society (a hazy argument but still, it can be made). I think we would have to say a contribution is an added positive factor. We can't just say that by not doing negative things, we are contributing. After all by that metric we could probably argue most convicted/imprisoned criminals are still contributing, and I think we can agree that once you are removed from society in general it is very difficult to still contribute to that society. So, contributing means bringing something to the table. I think at this point this is where the discussion of whether something is "good enough" comes into play and I think we should avoid that. If you are doing something positive that helps society, doesn't matter if it's art or science or curing cancer. All of these things can help people. Hell, garbagemen are contributing ENORMOUSLY to society by cleaning up all our crap... ...or are they? Is it still a contribution if you are being paid for it? Or at that point is it just a job? Is it a contribution if you could make more money doing other things elsewhere but you choose your career because it's what you love (because you feel you make a difference that way)? So my boss is here so I can't keep rambling but just some things to think about.
I see the point you make regarding a contribution to society and whether it's a contribution or a job, once money/pay comes in as a factor. I think most people would say volunteers (e.g. State Emergency Service, running homeless food drives/shelters etc) are most definitely providing a contribution to society, namely because these are people who give up their free time, without remuneration, for the help of others. But if there are other people undertaking the same work (say for the same group) but it's their actual job (thus receiving an income for their efforts), I don't think that should detract from the work they do, because of the 'being paid for it' factor. As an example, if a homeless shelter had 2 paid full-time staff and 2 volunteers, their efforts all make the same contribution. Regarding your other point re: choice of career, I think it should be at least somewhat admired if someone were to steer their career based on what they hope to achieve, and hold income expectation as a lesser factor.