> This is one of those very rare cases with potentially huge negative effect to make it into a mandatory feature.

The only thing you would need to make mandatory is a direct login to any storage service.

> Which still suffers from inefficient incremental updates. Curious whether those other managers deal with that (do they split your vault into parts)?

If it's a megabyte, who cares? You could have a way to split things or upload an update log, but it's not worth it.

If you need big attachments, then keep them outside the core vault file.

> Whatever documents you want to securely share with others

Sharing is where a dedicated service is useful.

But if a feature like that needs a paid service, it should be an optional upgrade.