I would argue modern MacOS is not “bad software” per se, it’s just nothing to write home about. Back in the heyday you describe, it was innovative and quite spectacular compared to the competition. Nowadays it’s rivals are better featured in many respects, but it still does everything it needs to.
I’ve been running one with a dozen or more users on bare metal at home for the last two years. A little bit of spam but otherwise fine. No deliverability issues or anything.