You now have e2e crypto for files, so the next logical step is to add it for contacts & calendar, as these are (also or, maybe, even more) sensitive data!
Then the web interface wouldn’t work anymore and that is very important for those features,
Yes, I understand that. But you also accept feature loss for the files e2e encryption, so this is totally not a problem. It should be the user’s choice, after all! Make it optional, and I would gladly appreciate this feature loss!
And of course this encryption may not affect all contacts. You could also make it selectively, or, at least, it of course won’t matter for contacts received from other servers/usernames on the NextCloud instance or mails, which are used for sharing files. (so these are saved on the server anyway)
it would break compatibility with every application out there (no way your phone could use it unless we built a unique calendar app
That is not true. E.g. on Android it does not need yet another calendar app, just one app, which syncs the stuff. Same for many other OSes.
A good example is e.g. this app:
It e2e encrypts all contacts & calendar, and integrates with many platforms. The Android app is e.g. based on DavDroid, so you can do that, too.