> If you start having carriers wholesale-blocking calls based on their lack of STIR/SHAKEN's verifications, then you will completely disable the vast majority of IP- and POTS-based phone systems in the country, many of which were purchased/installed in the early-2000's.

It's far too early for carriers to block by default, but consumers should have the choice. If I could set my phone to give a busy signal to any caller not authenticated through STIR/SHAKEN, I would in a heartbeat.