Religious broadcasters have always been opportunists seeking the paths of least resistance to saturate the airwaves with their content. They did it on shortwave,
Other than Transworld Radio and HCJB, and a few local/national SW religious stations, the proportion of government run shortwave to religious group owned ones immensely favors the government side.
then AM radio, then FM translators, and now they're systematically taking out some of the best and most powerful FM signals.
And they are "taking them out" because nobody else wanted them when the seller put them on the market.
These organizations have been like a cancer on each radio band they've infiltrated. In pretty much every case, you could make the same point you just described, that the old operators found it increasingly difficult to be profitable so they sold out to the religious broadcasters since no one else wanted to step up. So while religious broadcasters don't initially cause the death of the band, their growth is a result of its sickness.
If you look at shortwave, which is way beyond the end of its lifecycle, you can see that the religious voices just tagged along with the big government propaganda operations that populated all the international bands (I exclude the "tropical bands" as they were always used for localized national coverage, not for international listening).
And international shortwave was never advertising supported. A few intents to do that died more than a half-century ago. Nearly all shortwave international broadcasting was government operated.
Then, when they eventually take over most of the band, a majority of the former audience who doesn't want to hear that kind of doctrine on every station abandons it.
Again, AM has mostly died due to a number of factors unrelated to religious broadcasting. First is the fact that nearly no large and medium market stations today even cover their own market fully; an example is Cleveland where only one AM approaches adequate day and night coverage of the metro survey area. Second is the general abandonment of AM for all but talk formats, leaving many, many stations with little else to do other than foreign language or religious options.
Shortwave is now completely dead. AM is well on its way, and we now see the same symptoms occurring at FM. Broadcast radio is very sick right now.
And this is a case of your trying to put the cart in front of the horse. Religious operators have come along to pick up the broken pieces of AM and the unsalable stations on FM because those stations had no way to be profitable.
The real issue is that the two youngest generations don't use over the air radio much or at all. The problem with radio is not a few religious stations in each market, it is not enough listeners and not enough revenue for commercial stations.