Then you say: "The sad part comes when some bright, young up-and-comer leaves off their interesting personal projects in less used languages in order to focus on only mainstream stuff."

Why is that sad? It's just like you said, "Neither path is right for everyone." I was bright and young once and had an opportunity to go into a start-up and shape the world, but I turned it down for a mainstream job, and I'm happy I did.

There is no reason to pity my choice or mourn for me. And there is no reason to continue to foster the mentality that anyone who has never worked in a start-up is some sort of lesser developer.

Too often IRL and in communities like this I come across people who are obviously wistful that the apparent "correct one-size-fits-all career path" means doing business stuff in Java, C#, or whatever.

If you realized your options and went mainstream then you aren't who I meant. I apologize if I didn't make that clear. I've also done mainstream business stuff, and with a good company it can be fulfilling work. I certainly don't mean to denigrate that choice or those who take it.