As browsing any discussion on recruiting and interviewing with illustrate, hiring is already fucked up, and adding age plus a multi year gap would make getting a new job quite hard, let alone one that matches the compensation one has today.
I know a guy who FIREd a while ago, long before we called it FIRE, and he's had a heck of a time landing another corporate job. He had a very successful business development firm and took a buyout from partners so he could move away from NYC and spend time with his kids. His kids are all grown up now and he's been looking for jobs and...crickets.
But I agree in general. If someone retires at 40 and realizes 5-10 years later this isn't working out, that's a pretty big uphill climb for conventional professional employment.