I agree that a true downgrade attack would be a very serious vulnerability in 4G, particularly one like I alluded to that could take the RRC down to EIA0-- but

I agree that a true downgrade attack would be a very serious vulnerability in 4G, particularly one like I alluded to that could take the RRC down to EIA0-- but that isn't what's being discussed here, and calling it a vulnerability in LTE is inappropriate, just as calling HTTP fallback in browsers a vulnerability in HTTPS would be inappropriate. The vulnerability, to the extent it exists at all, is on the client, in this case mobile devices and in your example the browser.