Common interpretation of the term "sabbatical" seems to imply that there's something wrong. But there's nothing wrong with simply not being employed for 50 years without more than a week or two break in the monotony. If anything, the opposite may be true.

… or the concept that a day need not be filled at all.

Most common question i got from friends: “what are you doing all day?”. 

They can’t imagine because they lack the curiosity to fill a day with books, learning, sports, etc…

I agree! 

Some of the smartest people I know took 1 year off at some point

100%! It’s status thing, if you’re not ‘busy’ you’re doing something wrong.