I think you missed the point. The costs isn't insomuch R&D, it's in support. Users struggle with SSO and so we get tickets; techs answering tickets costs money.

On the other hand you have dev time that needs to save user passwords in a secure way. And this is the part of software that requirements maintenance and updates.

Sure, other software needs that too. Some enterprises use their own identity provider and I would prefer that as well. Usually that is probably some windows ldap database that can easily be integrated, but there are a lot of other solutions as well.