This is a mostly inaccurate trope from every time I've gone looking. If you look at the median and spread of salaries for Cobol developers or mainframes or Fortran or whatnot, it is way below the medians and spread for salaries of frontend and backend developers.

Now possibly there are non-public markets where you can hire devs at crazy salaries but us not being able to find them also indicates the relative scarcity.

So overall no I don't think finding work in old-school tech will pay better or even have much better work-life balance.