I've been helping some of my friends who have CS degree but have been working in Mainframe for over decade with course materials for learning full-stack development, It's been hard for them to empty their cups. Migrating from COBOL to a high-level programming language seems like a tall ask and one needs tremendous motivation and environment to make it happen.

I agree that it would be comparatively easier for someone struck with Java or .Net legacy systems, But they would still need to re-wire their brains to make a successful switch.