I'm not sure about the subscription model. Everything is a subscription these days and I think it's a bad thing because if you've already made the icons then just set a fixed price. Made new icons? create a new version and charge for it.
For comparison, we pay Hoefler a similar recurring fee for using one font on our website and in our app. I think it's fair. Designing good fonts or icon sets is hard work, and a few hundred euros a year for what's a fundamental part of our marketing and image is a pretty good deal.