I think you’d be working to add quality to your backups for free. Sure, they’d also give it to other customers, but what matters is that you have it (and henceforth do not have to maintain your own fork).

I think the point is more that you'd be giving away work for free that is only usable by people paying someone other than you.

That's a more generous way of looking at it and certainly a fair point.

Isn't this argument basically true of any major open-source project? There's always a commercial user somewhere who benefits from your work without paying for it.

A contribution to Bitwarden would benefit the paid hosting, sure, but it'd also benefit folks who are self-hosting.

Well the clients are still quite nice and you can use them with VaultWarden for free and get attachments.