There's a large difference between "startups are not risky" and "corporate jobs are not risk-free". I don't know that I'd call being "prematurely specialized" a risk; it might reduce your total career potential, but if it keeps food on the table and a roof over your head, that might be fine for some people.

No job is risk-free, but startups are inherently risky, and moreso than most other forms of employment.