I see your point with the difficulty of classifying FKTs and yes they can get obscure pretty fast aaand they aren't contested all that often. However, I think this brings up an interesting point; are we talking about 'competitive races' or races that have the most prestige? I think it is very difficult to tease these two out in this MUT world.
Setting an FKT can land you a story in outside magazine or make you the star of a film at a mountain film festival, both of which have a greater reach than Trailrunner, Irunfar, or Ultrarunner Magazine. Sure these efforts aren't as competitive, but a lot of 100 milers or other ultra races aren't either.
Because there are so many races, and so many different types of races(distance, terrain, etc) we will always be speculating a bit as to what races are the most competitive. 100 milers, and I suppose 200 milers now, captivate people's imagination and as such the ultra world will always place more emphasis on these events. They also start to require a different skillset than shorter races do. However, outside of a few hundreds, many of these races simply aren't all that competitive compared to shorter races. Typically, the most prestigious and lucrative races attract the best talent, but because many athletes don't want to run 100 miles these prestigious events often aren't as hotly contested. Yes races like Western and TNF 50 boast deep fields, but other well covered events such as Hardrock and Leadville often have pretty soft fields.
With regards to the AT PCT comparison, the AT record has gotten a lot more attention than the PCT with Meltzer and Jurek making appearances on Good Morning America etc but they are probably equally as competitive.
I get what you're saying about economy on different surfaces, but I think you're misunderstanding my point a bit. Undoubtedly it requires a strong aerobic engine to run a sub 4 minute mile, however, there are plenty of athletes out there who possess a similar V02 max but will never run that fast because they don't possess the mechanics or the economy to do so. My point is simply that some athletes might perform better on the trails than we would think ( Zach Miller or Tony K) because they have a very strong aerobic engine, where as others might not perform quite as well as one would expect because their ability is due to amazing economy. While successful road runners do have good aerobic engines, it is this engine combined with their ability to move correctly and efficiently that allows them to excel. In sports such as cycling it is largely just the aerobic engine that determines how fast someone can ride up a hill. Trail running is a bit more dependent on the aerobic component than road running. For example, I know a few Skimo athletes who can beat up on some fast runners in a VK but would get their asses handed to them in any race that requires 'running.' To be clear, I am not saying that road/track athletes can't or don't excel in the trail/mountain world, I am just saying that athletes who aren't very talented on the flats might excel in steeper terrain if they possess a big aerobic engine.
Unfortunately, Skimo races in the US don't have a ton of ridge scrambling in them, save for a few steep bootpacks most of the technical aspects of these races has to do with the difficult skinning and skiing. But if you do actual ski mountaineering, like really go ski some gnarly lines, then yes, that will definitely help to redefine your definition of what is technical or steep. The ability to apply a ton of aerobic stress and the relatively similar motion to running (or perhaps hiking), seems to make Skimo a pretty great training option for mountainous running events though. Francois, Killian, Emelie, Ida, etc seem to suggest that this sort of cross training is a great tool for trail runners.