I would really like outdoor rides without power data to do a TSS estimate based on heart rate. I know heart rate is far from perfect, but it’s way more accurate than subjective 1-10 RPE.
Subjective rating requires a mental average of effort, which people aren’t great at. Most outdoor rides (especially MTB) are pretty variable. I always struggle how to rate the effort, especially on longer rides where the difference between a 7 and 8 is a big leap in TSS.
Yes heart rate is affected by things like weather and long term fatigue, but so are subjective scores, so that is no reason not to calculate from heart rate.
I would think calculating a rough TSS from heart rate would not be that hard and is going to be better at helping riders keep weekly TSS on track while cross training (gravel, MTB). Cross training keeps me from burning out on the road/trainer. At the same time, I can’t justify a power meter on every bike.
Also, this approach would be helpful for making TR more attractive to triathletes, since it would be useful for swim and run workouts.