I can't believe I haven't noticed this thread until now. What Kirk is proposing sounds familiar to WAKY's setup in Louisville. 103.5 rimshots the city from the south, leaving downtown with a poor signal. (See
this longley-rice map) They also broadcast on 620 AM, from 6 miles east of Downtown, with 500 watts, directional to the west. I'd imagine this signal isn't exactly penetrating, however it provides better signal to those office buildings than 103.5 does, and there's an added bonus of Bloomington reception. Interestingly enough, WAKY also employs a 100.1 signal in downtown to help bridge the gap on FM, and they are also on 104.9 and 106.3 north-east of Louisville.
However, as others have noted above, AM does not have the best sound quality, FM does. (This is somewhat subjective because I'm more willing to put up with music on AM than others here, however I will absolutely say FM sounds better). Not to mention, we don't really know how long the band has left until it becomes a breeding ground for all things Brother Stair, like Shortwave.
In a world where FM hadn't really taken over yet (probably looking at the 1960's here), you might have a good strategy. People, still accustomed to listening to AM, are surprised that their FM station doesn't travel nearly as far as the AM, and after all, this was before FM went stereo, so there was only a minor improvement anyways, might as well go back to AM! However, as Kelly pointed out, there are options out there that doesn't involve AM, and you get to keep the sound quality.
I agree with what Mr. Berry said about AM transmitters requiring all of their power to achieve full modulation, however the fact remains that AM can just cover more miles. In his example, 5kw+2.5kw is 7.5kw, but if that station is on some good flat land, on a low frequency, it can top 200 or 300 miles easily, whereas the FM station is limited to the curvature of the Earth and the height of the local terrain (usually resulting in 60-120 miles of coverage, however, an FM signal in space can travel quite far without weakening).
An interesting comparison to Mr. Berry's point is 1240 KFBC at 700 watts, and 870 KJMP at 1,200 watts. It appears that KJMP failed to do the mathematics here, because their signal is meh even in the Fort Collins area, however KFBC was actually stronger than KJMP during my trip there, despite lower wattage and higher frequency. AM can go farther than FM with lower wattage, but it must be taken care of properly.
To some of your points, I kind of noticed that I will listen to the FM translator of the station if I'm in range, however I take comfort in the fact that if I drive out of the FM's range, I can sit on the AM for a long shot before it becomes unlistenable. For example, KNX-FM (formerly KNOU) covers LA pretty good, but if I wanted to listen to it up north in Victorville, or south in San Diego, I would be out of range, unless I flipped over to 1070 (or you know, streaming, but just for this point, I chose 1070), and I would still have some room to travel before I would deem the 1070 signal un-listenable. Of course, AM's useful range today is limited because of RFI noise, however when you are just raveling in your car, your chances of running into overwhelming RFI is maybe 15%, and I actually put that in practice recently with KGAB. You can listen to 99.5 for a good 20 miles east on I-80 before it really starts to have problems, but once it does, 650 can carry me another 100 miles easily. Similarly, 99.5 is good on I-25 until you reach exit 4, and then KQMT just takes over, but 650 can last me all the way into Denver, at local strength, and if I did tolerate some static, it could potentially make it to Colorado Springs.
To PT's point, AM land plots can be extremely huge at times, to maintain the ground systems, however a station higher up on the band doesn't need as much land, and my local 1630 operates with only one block, and half of that block is already occupied by a store. It manages 10kw and covers at least to Brighton with a listenable signal, with only 1 little parcel of land here. On the other hand, some stations sport a massive array, like WLS, KCBS, WFAN, and KDFD.
Red Plume puts it perfectly for me. The only thing that really counts on AM is distance and coverage, and without the distance factor, then why be on AM?
Kirk, you do have the right idea at heart, but implementing it in 2022 is kind of odd. However, you are always free to try it out on your own stations, or convince an AM to give it a try. Your strategy does have some basis in what others have done, but it hasn't been tried like this before! I'm a big fan of terrestrial radio (AM/FM), but I suspect it's time to give up on that, and even Sirius at this point. All today's folks want is more bandwidth for my 5G Phone with 4K video! YAH! Even if they know that their cell-coverage is bad anyways