I don’t think this is as mutually exclusive as you imply.
Good COBOL programmers are expensive because they’re rare, and the only way to become a good COBOL programmer is to spend a decent fraction of your education and/or career working with it. That doesn’t happen organically anymore for any significant fraction or junior devs.
>Yes, exactly. If you can find a way to get good enough to do it, you're guaranteed a great income.
You can't replicate years or decades of Cobol project experience to "get good at it", out of thin air by doing some side projects at home. No amount of individual self study can prepare you for industry specific cruft and issues you've never encountered. If it were that accessible, a lot of people would do it.
Good COBOL programmers are expensive because they’re rare, and the only way to become a good COBOL programmer is to spend a decent fraction of your education and/or career working with it. That doesn’t happen organically anymore for any significant fraction or junior devs.