David,
I'm not sure which propagation software package and what version of Census data you use? See the attached coverage map of WMKL, as currently licensed. This shows the 60 dBu and 54 dBu contours plotted with Longley-Rice propagation model and 3 second terrain DEM with the 2017 Census ACS 5-year estimate data.
Census data updated to 2019, and the propagation is using the simple projections that the FCC also uses. You can see that data at a variety of sites, such as radio-locator.com and https://home.recnet.com/
That is the data used by the FCC for licensing, not the Longley-Rice models.
With the terrain in South Florida, the actual in-vehicle coverage is well out beyond 50 dBu - there is WMKL reception well into Palm Beach, Collier, and Monroe Counties. There is great WMKL indoor clock and table top radio reception even less than 60 dBu (Cutler Bay, Palmetto Bay on the coast). Where are you listening? Indoors or vehicle?
I've had contact with, programmed and managed stations in Miami off and on since first meeting Todd Storz at WQAM back in early 1964. I later consulted WQBA, managed for Metroplex, consulted WSUA, and created the WAMR format in '95 and was involved with the Heftel, HBC and Univision stations up to, well, now!
I've lived in Hollywood, Coral Gables, Kendall and at the Trail nearly at Krome, so I am very familiar with the market and how signals propagate.
One experience was with an FM on the Channel 6 tower where we had the opportunity to see how much listening we got in-home and at-work at the northern fringe of the signal... just as I said, 95% was within the 65 dbu coverage area and 80% in the 70 dbu coverage area.
I don't think it is realistic to assume that the majority of radio listening still happens from home in 2020, at least not in South Florida with lengthy drive-time commute times.
Over half of listening in normal times happens at fixed locations, home and work. In home listening is very much restricted to at least 65 dbu coverage.
But to prove that, back when we had home, work and car segregation in the Arbitron data, we analyzed over 5 years of data for every one of the top 10 to 15 stations in a dozen top 25 markets including Miami; the data included millions and millions of quarter hours. That is where the information on signal vs. actual listening comes from.
The majority of households (ages 18-34) do not have a radio at home.
That's not true. The figure is slightly under 30% of households do not have a working radio at home.
Unfortunately, the trend of AM stations disappearing is inevitable.
And that is because few cover their whole market. How many Miami MSA AMs cover 80% or better of the market day and night? You can count them on the finger of one hand and have fingers left over.
The FM in Key West (WJIR on-air for at least 35 years) blanketed Key West. The LPFM (WORZ-LP) exclusively served the affluent Ocean Reef Club community in North Key Largo with a local signal.
Monroe County is the USVI of the mainland. No industry, no real commerce other than retail and tourism. Low average household income... and about 5 times too many stations. Gotta' be the worst radio market in the US.
The decline of some FM stations has more to do with other factors than signal or coverage. People are just consuming media differently than they did 10-15 years ago depending on their age and unfortunately that is less audio consumption of terrestrial radio.
Radio is still healthy in many markets (excluding the current pandemic) where there are not too many stations. And in all markets, the top stations do well. Like a horse race, some "good ones" finish last or next to last.
Put it this way: if your station were on a commercial channel and you gave me the facility for free along with $1 million in cash for initial operations, I'd turn you down. Alligators make lousy pets and even worse listeners.