If you have an employer, this is usually a pretty easy sell. Approach your boss about transitioning Aging Product X to a new technology stack because the current one is becoming defunct. Of course this will require some training for you, but it is easier to teach you a new skill set than to hire someone with that skill set and get them to learn about Aging Product X.

If you aren't employed and are contracting/freelance, it is just as easy. Since your skill set is niche, not everyone has it, and you can charge more for that niche. Up your rates enough that you can cut 10 hours a week out to study something new.