Another example is calendars. From what I can see, you can't share a Proton calendar with a non-Proton account. If I went to Proton, I would probably need to keep Google Calendar to sync calendars with my spouse. I'm more concerned about tech stuff and privacy things than she is so I just don't think I could convince her to move. And I think the price for Proton Duo is just too high for us.
2/6