That's true, but you don't need hundreds of thousands of alternate companies. You're reasonably secure having 10+ that you could jump to. C++ is fine in that regards; where it becomes a problem is careers like aeronautical engineering or petroleum geology where you have a really specialized skillset that's used by only 2-3 dominant companies.

For finding a niche - basically you try out a bunch of different careers, problem domains, and skillsets, and specialize in one where a.) you enjoy the daily work involved b.) the people around you tell you you're pretty good at it and c.) you get paid reasonably well for it. It can take a fair bit of job-hopping and exploration to get there. Do this at the beginning of a career when it's expected of everyone.