MCV1: Some of those cracks esp on the corner of the tread look like the zone where a bubble might develop. The "T" on the Firestone looks like curb trauma (even though you say it's never happened...). But still, you make a good point. Looks like your tires are getting down there, and living in WY, it would probably be wise to replace them. Even though the sidewall can look ok, once they start to dry-rot, the sidewall is the thinnest part of the tire.
Of the tires I've had fail from dry-rotting, Michelin has stood out as being the worst. If it's a DD, they should last till they're worn out, but if it's a 3rd or 4th vehicle that doesn't get driven as much, they'll likely dry rot to the point of being unsafe in about 6-7years if left outside (maybe sooner). Maybe a little longer if indoors. That comes from owning a few sets of them, not just one. These days with more people working from home, its something to consider when buying tires. I had some Firestones that were unsafe at ~12 years. But I've had Coopers which looked great ..rubber still supple at 15+ years of sitting outside! And multiple sets of Uniroyals too where the rubber held up great. The Michelins and Firestone's I've had really ran out super smooth, so not knocking the quality of product when purchased. Just the long-term dry rotting cut their lives short.
On a snow plow Jeep, I had 6 exact same Goodyear Wranglers SR-A's (235/75/15) on aluminum Jeep wheels. Great tread, worked great as a snow plow tread. They started dry rotting in a few years and at 7 years or so they were looking not so safe. I put them on the plow Jeep because it doesn't get driven on highway miles. They lasted till about 10 years. But here's the thing, 3 of them the sidewalls tore while plowing snow. I suppose the rubber being colder and more brittle and under torque ...in the cold, it found the weakest spot. They all tore within a fairly short time on the inner sidewall which didn't look bad at all. I had a boss who had an old tire (on a classic car) explode and damaged the rear quarter.
So I've seen, dry-rotting is a real thing and shortens a tire's life. I shy away from tires that advertise "high silicone compound" anymore, but who knows what compound is used in which tires these days? What compound was used years ago is probably not the same today. What compound is used in one tire may not be the same as another tire, made at another plant, in another country. But that said, the Michelin/Firestones/Goodyears I've used were a fantastic product for DD use besides the dry rot issue ...which was mainly an issue because they were on seldom-driven vehicles.