There is fundamental opposition to the very notion of a national ID system in the United States, it's not as simple as "opt-in" or "SSN would be backwards compatible."

Here's the ACLU's stance: https://www.aclu.org/other/5-problems-national-id-cards

I didn't suggest a national ID, I suggested optional 2-factor SSN. I'm well aware of the spectrum of concerns of a national ID system.

This, but include an actual SSN sunset date with a PKI-style replacement in 10 years.