When a technology is less popular it's hard to find a developer, you need to pay for relocation, etc. Salaries go up but job security is down. You want to change jobs and you're stuck.
You should very much learn functional programming as well as OOP and give yourself a well rounded education. But I would suggest correlating the salary information with the popularity information and the job listings in the area where you want to live.
When a technology is less popular it's hard to find a developer, you need to pay for relocation, etc. Salaries go up but job security is down. You want to change jobs and you're stuck.
You should very much learn functional programming as well as OOP and give yourself a well rounded education. But I would suggest correlating the salary information with the popularity information and the job listings in the area where you want to live.
The best salary information that I have found so far is levels.fyi and the "trimodal" article.