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.

Yeah I would generally agree to that. I got mine for £600 and admit it's more than I really needed. But it's a pretty nice bike and I use it a lot. It was the cheaper model from the more expensive brand so my thinking was it's probably a pretty good frame and I can replace other parts as I go.

Replaced the tyres after about 2 years, new ones cost like twice as much but marathon plus touring tyres should last far longer and are much more puncture resistant.

Everyone's finances are different, your economic guardrails won't necessarily make sense for someone else. I've built plenty of commuters that have rolled out the door of my local coop for < $50 and I put love into every single one of them, every single one of them was (Mostly) solid. But I also have a $3.5K dream build that mostly just hangs on my wall made from vintage new-on-shelf parts, an ode to late-90s / early-00s roadie aesthetics.

I do agree, but in her defense she is making full use of it and over time has progressed from the basic commuter to more advanced bikes which have had significant increase to speed and enjoyability/comfort of the ride. Up until now she has already rode 5k km just this year, planning to finish this year with 10k km.

For everyone else I do agree and have a basic commuter that I literally got from a trash pile through her connections to local biking community.