> Here's a simple one: Build an open standards-based system where users choose their backup provider and let them decide whether they prefer to have full control over the encryption keys or whether they want them to be managed by a third party. Educate them so that they can at least try to make an informed decision. Warn them about the risks and jurisdictions of built-in providers.

WhatsApp performs (on Android) backups inside a normal directory. You can just backup it via e.g. Syncthing, which I've done in the past.

It doesn't get much more standards based. Strongly pushing users to deviate from the standard flow IMHO risks them quickly choosing poorer options (e.g. a free, dubious, online hoster). At least with the current setup, the "easy option" is quite safe in terms of not loosing data and very low risk that anyone apart from you and governments with lawful access ever accesses the data.