Trouble is if your company pivots and you have been doing Mainframe COBOL for 20 years - you might find transitioning to a new language hard.
Now? I'm stuck working for a salary in a job that pays the bills. I live in Texas (I know, I know) where IT salaries are already low. Texas has that "right to work" mentality. My bosses are not real IT guys in the sense that they love IT. They're in it for the salary, whereas I'm in it because I still love it after a span of pushing bits over three decades. I would retire only if I won the lottery, but if I did that, I would likely buy a Z-Series mainframe and spend my days playing.
Then you are not following the advice discussed.
At least, that's my interpretation of the article's logic mixed with what appears mainstream to me. There's a lot of room for subjectivity, but it's difficult to argue that Java/Android isn't an extremely mainstream direction.