After using Google Calendar for years, it is time to change. I don't want to host and maintain my own calDav server, but still have control over my data. Blockstack is a good solution and OI Calendar the new interface to my calendar in the cloud.
There was a bigger campaign back in 2015 to De-Google-ify your life. The alternatives for Google Calendar back then were for example any.do (which turned into a paid task manager service) or tempo.ai (which does not exist anymore). Framasoft, main driver of this campaign in France, offers framagenda.org, a service based on nextcloud, a hosted calDav server. While framagend is a great product, it still makes you dependent on the company running the servers for you.
OI Calendar is different. You own your event data. If OI Calendar does not exist anymore, you have your data and you can even take the code and just continue (once Blockstack has the migration feature between apps finished).