You seem to be saying the same things as I did. My point was that is very unusual for a small business owner, to sell a profitable business because it's not taking up enough of your time.
The reason a lot of people want that is they want a passive income to give them security.
Given the success of instapaper and the fact he'll have done fine out of Tumblr, I suspect Marco has no need for that and would rather devote his time and attention (something can be distracting even while taking little actual time) to something he'd be happier doing.
Under him The Magazine as a product probably would stagnate like Instapaper did. Then he would have sold it anyway.
And, advisory role usually means a decent chunk of shares too. With what Glenn have in mind, this might be a smart financial step.