> Now compare that to the PSTN: what does 555-123-4567 really tell you?
It tells you exactly as much as the "from" field does in your email.
> you have more tools to work with in email.
Only if you're an engineer implementing a mail server configuration. But if you're implementing a telco you also have more tools to work with than a caller ID.
End users use DMARC/SPF the same way end users use STIR/SHAKEN... they don't. None of them are user-servicable values. And service providers use DMARC/SPF the same way end users use STIR/SHAKEN... they implement those controls for their users in the form of a managed service.