And where do you get a modern road or gravel bike for $600 or less? Even used ones are often more expensive.

A nice road bike is a lot of fun and paying $2k for it is similar to buying a gaming pc. No one really needs it but sometimes it's ok to spend money on having fun. And even $5k is not a bad decision if you have the money and value fun more than money. My road bike only cost me 700€ (plus all the additional cost for new tyres, saddle etc) but in the next couple of years I will definetly buy a second one that will be more expensive.