The point is not about limiting it to AGI, the point is setting limits on what data is shared so that you as the consumer always control what a requesting company sees. Don't want your data shared? No problem, you can set to deny all requests. Only want to share AGI but not your entire return, just allow that field, etc.
Correct me if I'm wrong, but your response also seems to indicate you don't work in financial services. No company is providing credit based solely on income verification (well, not that I'm aware of maybe some fintech is doing it). Most all credit providers are collating data from various sources to make their decision and it varies more than you might think on what goes into a credit decision.
Further, I don't think there's any need at all for laws for this as you suggest, rather just let the IRS offer it as a competing service with private industry. I know many companies that would sign up right away for such a service at the right price point.