Calendly is a good product. But I don't believe it has moat or a unique offering.

Identical competitors is popping up easily by offering the exact same service. It is a simple concept and the fact is that the only innovation they can add is adding very simple bells and whistles. The barier to this industry is virtually non existing.

Nothing is stopping Google or Microsoft to build their own integrated calendly like app. Considering platform risks, I don't believe any SAAS whose core offering relies on external API to have a sustainable revenue model.