To me, the most important point is not the semantic difference between "profitable" and "sustainable" but instead the author's invocation of opportunity cost: "

To me, the most important point is not the semantic difference between "profitable" and "sustainable" but instead the author's invocation of opportunity cost: "You could have rented the space out at fair market value for much more money than that. I'd argue you're losing money." While it's nice to have the security of an infinite runway, if the value created by staying on that path is less than following a different path, then overall it's a net loss.