One project I've been working on is a competing (or now no longer competing :P) platform called https://classalog.org (marketplace of in-person classes). Found the same thing: buying paid ads does not make sense for a platform – you need to do this with free traffic and referrals.

I think the kind of math above is useful, but it doesn't replace making something people want. Maybe to get a rough idea of the metrics, sure – but airbnb etc wouldn't have started either if they'd done the math above.