In this case, knowing WP might be worth it.
Besides the excellent answers by people above, let me make a distinction regarding WP developers.
The first kind, are those developers that makes their own custom theme, custom plugins, and might even sell these themes and plugins to other developers. That could become a lucrative income stream. For example: Elegant themes have 430,000+ customers, selling themes at either $89/yr or $249/lifetime for access to all their 80+ themes. Do the math.
The second kind are the WP end-user developers. They setup WP, customize it, (maybe even make their own theme), and sell their service to small business owners/companies. Once the site is built, they hand over the keys (Id/passwords) to the company and they go find another client. They may charge hourly rate, or a fixed rate per project.
But there’s no denying that the field is crowded, and you’ll be competing against people living in different countries where standard/cost of living varies. So a person in country A may need to charge $1K for it to be worth it, while a person in country B may just need to charge $200 for it to be worth it. If you’re living in country A, it will be hard to compete against people living in country B… unless, you develop a local reputation and target local businesses in your community (as opposed to competing in Fiverr). These local businesses in your community, the people living in country B won’t have access too… and vice versa.
Now, about working in a startup, or a company providing enterprise solution… will knowing WP be a good career path? Most probably Not.