@ggg17 I'll try to address all of your points.
There is a point of plateau, hence the need for structured training and variety. I feel this is addressed in my post by advocating for use of several methods, as well as incorporating progressive overload and exercise variety.
I am referring to the rep range continuum in regards to training for hypertrophy, not whether it is useful for understanding the difference for training strength vs endurance. I do not think it is useless for that, but people have extrapolated the strength-endurance continuum to make simplistic assumptions about hypertrophy training. It has been a source of confusion.
You don't need to be a professional athlete to train outside of a given rep range, nor do your workouts need to be any longer or more frequent than they would have been otherwise. In fact, certain methods like the use of myoreps and shortened rest periods are extremely time-efficient, and are more than sufficient to offset the time it may take to complete more reps. Even if the sets take slightly longer, the distinction is arbitrary.
Example: 3 sets of 8, each rep takes 3 seconds (just making a rough guess on how long a rep would take), 120 seconds rest between sets. 8 reps x3 seconds = 24 seconds, multiply that by 3 sets to get 72 seconds total. Add 3 rest periods of 120 seconds (360), you end up with 432 seconds, or 7 minutes and 12 seconds.
Now compare this to 3 sets of 20, each rep takes 3 seconds, 90 seconds rest between sets. 20 reps x3 seconds = 60 seconds. Multiply by 3 sets and you get 180 seconds. Add 3 rest periods of 90 seconds (270) and you get 450 seconds, or 7 minutes and 30 seconds.
So the 3 sets of 20 with 90 seconds rest takes 18 seconds longer than the 3 sets of 8 with 120 seconds rest. An 18 second difference in 3 sets is arbitrary and is not enough time to add significant length to a workout.