> Unfortunately, the rep insisted that all telcos would follow suit, and there would be no phones used without 5G compatibility soon.
This has approximately zero chance of being true. For instance, all iPhones before iPhone 12 lacked 5G. There's no way AT&T or any major carrier will get away with nuking support for iPhone 11, XS, X, 9, or 8. Probably not even iPhone 7, which still supports the latest version of iOS.