I gave up on ProtonMail. The lack of a calendar means you often need to go back to using Google Calendar or Outlook.com Calendar, kind of negating the privacy benefits if you're a heavy calendar user. Secondly, its been years and you still can't store more than a single email address for a contact. This is so incredibly ridiculous that I have an extremely hard time understanding how they get away with charging what t…
There are many business, coordination, and communication tools that could be in my mail client (I do a lot of billing over email, is an email provider unusable if it doesn't integrate Quickbooks-like functionality too? What about shipping/receiving, project management, phone, SMS, mapping, etc, all things related to my use of email?), but I don't think they need to be there.
Calendar protocols are not federated like email. Choose between Exchange, Google, and iCloud. You'll have an easier time if you use the same one as most of your contacts.
Choose the best tool for the job. It's not likely that one provider will have the best tool for everything that, say, Outlook does.