You contradict yourself. First you say: "Neither path is right for everyone"

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.