Why have subscriptions (i.e., recurring payments) not taken off in web3?

Co-Founder at sablier.com, and open-source developer. I have a broad range of interests, including longevity, epistemology, physics, and psychology.

I think that a set subscription price is trivial to implement, but usage based subscriptions are technically challenging to implement as you introduce a source of trust that defines the usage in each period. 

Making this source of trust sufficiently secure, esp when that source is offchain, requires novel design