True, but I can feel a difference in a power threshold. I cannot feel a difference in FatMax. Some can feel a difference in lactate accumulation at lower power levels (i.e. LT1). So a power threshold has utility on a day-to-day basis, even with daily variations. IME, FTP doesn’t vary by 10% day to day unless you are very fatigued (in which case you probably aren’t training at FTP anyway, but you might be racing!). The variances in FatMax were even larger than that.
Hence the utility of power and calibration of RPE remains the most practical tool in the toolbox of most athletes.
Again, if the curiosity is purely intellectual, great! All for it.
I don’t know why anything should be interesting about the crossover point (assuming what you mean by that is the point where the 50/50 balance of substrate utilization occurs). I can’t imagine caring less about something.
But your point on your INSCYD test is bang on, and the one that needs highlighting for athletes before they go drop money on those tests: what the model reflects and what’s actually going on are very different, to the point that I think the INSCYD model is invalid and subject to massive errors. I don’t remember exactly, but I am pretty sure my curve during my trial test said my fat utilization dropped to zero at like 260W (coming in off my offseason where I had just tested 60-min power at 255W). And that’s just wrong.
From a coaching/performance perspective, there is nothing wrong with carbohydrate utilization (quite the opposite, actually), even on endurance rides. And as studies and practical knowledge show, you burn more fat by riding for a longer duration… so from a training perspective, stick that power well below LT1 (and probably FatMax) and sit there for 5 hours.
I think there is some utility in tracking LT1 over time (and probably MLSS/LT2/Threshold too), but I also think you can do a lot of that with a power meter and HRM (EF) and a good sense of RPE and what’s going on under the hood.