Friedberg was forecasting that by May–June 2021 enough Americans would be vaccinated that people would actually be flying without masks and feel comfortable doing so, essentially regardless of rules.

What happened:

1. Vaccination levels were indeed high by late June 2021. By June 27, 2021, about 54% of the total U.S. population had received at least one COVID-19 vaccine dose (about two‑thirds of adults). (naag.org) This satisfies the “enough people are vaccinated” part of his reasoning.

2. However, U.S. commercial air travel remained under a strict federal mask mandate. TSA began requiring masks across U.S. transportation networks (including airports and airplanes) starting February 2, 2021, initially through May 11. (tsa.gov) On April 30, 2021, TSA extended this requirement through September 13, 2021, thereby fully covering the May–June 2021 period. (tsa.gov) The mandate was then extended multiple times into 2022. (tsa.gov) In practice, passengers (except small children and a few exemptions) could not choose to fly maskless during May–June 2021.

3. Public attitudes also don’t support the idea that most vaccinated people were ready to fly maskless that early. A June 2, 2021 Economist/YouGov poll found that about 80% of vaccinated adults said they would wear a mask when traveling by plane, with mask‑wearing on planes broadly accepted across vaccination groups. (today.yougov.com) That suggests many vaccinated travelers still preferred masking on flights rather than feeling comfortable going without.

Because by May–June 2021 people were not actually flying without masks in the U.S., and available polling indicates most vaccinated Americans still intended to wear masks on planes, Friedberg’s concrete prediction about maskless, comfortable air travel in that timeframe did not come true. The vaccination part was roughly right, but the central behavioral claim (widespread, comfortable maskless flying by then) was wrong.