Seconding this. I have a single speed that I use for commuting in NYC. I've had it over ten years, I think it was $500. With regular maintenance it's been great. It's my only bike and I easily overtake "better" bikes on the bridges. I've biked century rides on it.

A city tip for a different aspect of longevity, as I only park my bike on the street. No brand name parts, or file them off/cover with band stickers, take off all logos on the frame, spray paint it in the worst way possible, hit the finish with a chain to cultivate a decent patina.

Actively make it look like trash and no one will steal it.