Imagine if the U.S. had such a requirement before Docket 80-90 was enacted. I'll bet that 90% of the upgrades and new allocations could not have met the type of sustainability that Canada imposed at the time.
I was able to secure a "dead" frequency in Quito where there were 42 AMs in a marke of just under a million people in 1964; a licence had no value because there were about 25 too many. Some billed less than U$S 250 a month plus trade for meals and rent and the like. By 1970, one owner had 5 of the top 10 stations, and nearly 50% of market revenue. Most of the rest did not show in ratings or contribute to any diversity in programming.
Our ongoing discussion of AM often does not include the fact that the vast majority of rated market AMs have inadequate signals, and, overall, many of all AMs are daytimers or "virtual daytime" with very low power and exist only to support and FM translator.
I wonder what percentage of AMs are viable on their own if they had not full FM or a translator?