An alternative could be a sort of Proof-of-Work executed by browsers via WebAssembly.
Similar to the now-defunct CoinHive service, but like, used for verifying that you are not a bot.
could still be bypassed by bots, but would be more expensive as you would have to get a CPU mining rig just to verify captchas.

another advantage of such a service is that you earn mining fees too.

Unfortunatly I am not aware of any such service.