To start, regardless of whether the experiment pays off long-term, that does not negate the fact that it does require extra design and development time. There is much more code to handle, regardless of whether you use a framework or build it bespoke. Additionally, whether you design for mobile first, or last, there is more design time required. Period.
Second, both the author and OP have lumped 100% of all websites together, and generalized the argument. Sure, responsive may be worthwhile in some cases (as the post misses), but there are many cases where it isn't worthwhile.
My argument for using responsive is that yes, it's worthwhile, if and when the return is there. As a freelancer, this means I must be getting paid for the extra work. As a company owner, this means that we must see value from the resource spend, which could be in the form of better UX, more eyeballs, or more profits.