You can still buy a SIM, a prepaid PIN, and a phone with cash, but you'd need to pay a non-correlated person to be seen on CCTV to do it, at a non-correlated time, and hope they don't just take your money and leave you nothing at the dead drop.
Then there's the hassle of setting up the account in a way that's not correlated with your location, normal waking hours, etc.
All of this could just be avoided if Signal did the right thing.
But they won't. Ask yourself why.
I buy all of my anonymous prepaid SIMs with cash at retail myself, and they are still anonymous.
The only time you’d need to stay off CCTV is if you were using them to commit crimes and expected a significant investigation to be undertaken.
Your casual assertion of malice on the part of Signal is not supported by any facts.