While I agree with you in general, E2EE can't be a blanket excuse for building bad applications. If your E2EE applications can't deal with data encryption in a way that makes them comparable to competitors non-E2EE products from a usability aspect, then they are still bad applications, regardless if we know that E2EE does make it all way harder.

I love Proton, but there are aspects of E2EE that they haven't worked around well enough at present, in my opinion.

The biggest pain points are:

    - Collaboration
    - Offline access

Both of those are highly relevant for calendars, so it doesn't surprise me they didn't hit the nail on the head on the first try.