Boats and cars depreciate in value pretty hard, so those are most likely sunk costs. Now mark has a cool boat and car, but that doesn't guarantee you a social network, much less a social network of professionals.
A doctor wanting to start a practice in the slip over would be quite a stroke of luck. An even bigger stroke that he would be any good, and yet even bigger stroke that he would want to start his practice with some random dude on a boat. All that for a shot at putting in the hard work so you can maybe have a successful practice.
In your scenario, it seems likely that Mark is actually pretty charming, and that's what would lead another boat doctor to want to start a practice with Mark. If so, then Mark probably doesn't need the boat or car to charm a peer from his existing network. Which would almost certainly be better ROI.
Additionally, even if everything went wrong for Sam, as you describe, he would have learned a ton for future cash flowing assets, and can probably sell them for what he purchased them for, possibly more if he improved the numbers. Between the knowledge, and the intermittent cash, Sam almost certainly has a much better ROI.
I would like to point out though, that I don't fully agree with the goal of the article. Sam seems like he is going to wake up one day and wonder where his life went. Doesn't sound like he is making any great new memories. Mark, on the other hand, probably won't be retiring in the Hamptons, but he will probably look back on his life and have made some awesome memories.