Add to that the non-existing costs of switching between these offerings. I was using the Calendly free tier, and when a feature I wanted was moved off the free tier, it took me all of 30 minutes to move to zcal (most time spent fixing web site, and email signatures).
Perhaps there's something I'm missing here. Maybe there's a social aspect that escapes me, but I would love to hear from someone building in this space about their motivation and long term goals.