Mapbox's free tier is an order of magnitude less permissive. They even charge for loading their JavaScript: $5 per kiloload! Feels like the best approach if I'm not trying to commercialize is just to encourage people to self-host using their own API key.

Assuming the service ever even gets to 2M tile requests/month. That alone would be quite the accomplishment.

Though with Gaia going to hell, something new needs to come along.