Pretty much agree with what you've said. I never had a thermocouple fail in the decades I had them. Never had a furnace fail to light and heat. The "new" furnace I had put in 13 years ago (Rheem) failed multiple times, once because the igniter failed, which can be quickly remedied if you keep a spare on hand. It's a 6 buck item on mine. Same could be said of a thermocouple.
The other times were caused by a flaky motherboard, and I always got it started by cleaning the flame sensor, though that was probably a motherboard related fluke too. Since the motherboard failed with a sticking fan relay and I had a new motherboard put in a few years ago there hasn't been a glitch with anything and I haven't touched the flame sensor. But he put in a the new flame sensor that came with the motherboard.
Besides it being almost the coldest day of year, the first time mine failed was on Christmas Eve! Lots of cursing about the furnace.
Anyway, I had this one put in, replacing the original pilot light/theromocouple furnace because I went to central air and had to replace the old one because of that. Went with an 85%, mostly because I was told the higher efficiency furnaces were even more prone to failure.
I recommended the OP think about replacing his furnace because it already has the motherboard and sensors, and is 15 years old. It's an option to think about if he can get good HVAC advice. But I didn't know what you said about cracked heat exchangers and carbon monoxide, since I had drunk the Kool-aid on that. So I kind of take that part back, though if you *do* have a leaky heat exchanger there's reasons to think about replacing the furnace.
When I had the motherboard replaced I asked the HVAC guy about getting the evap coils cleaned. He said something like "If the A/C is working like it always has been, it's not worth getting in the plenum to clean them. Wait until it's a problem."