Unpopular opinion - you may like to call it an SSO tax, but I think it's perfectly reasonable from both sides. The reality is - if you're a 10 person startup and the "SSO tax" is annoying, then simply don't do the SSO version...you have 10 people in your company, you can get them all to use a password manager with MFA. If you're worried about security then fine, don't you think it's worth paying a little more?
If people's issue with the "SSO tax" is that the SaaS software provider is making incremental money for very little effort/investment, then I would love to explain how the economics of most SaaS tenancy models work with regards to infrastructure spend...