I don't think its fitting that certain routines or training styles get referred to as 'volume training'. Volume isn't a training style or philosophy or routine, its a variable. I've stated before that total volume relative to intensity (percentage of 1RM) is probably one of the most important factors in a routines success or effectiveness.
I've never really felt that failure leads to more hypertrophy, I actually believe that the benefit of taking a set to failure comes from the fact that effort needed to complete the set is higher. Which means higher motor unit recruitment and will promote faster / quicker adaptions in strength. But obviously this would be impractical for most movements unless you train with a partner. In most cases its not necessary, even for bb'ing imo.
Plus it creates more challenges for programming. Managing all the fatigue and stresses caused by taking your main lifts to failure would be a real headache. That's why most 'failure oriented' routines take a single set approach. I am not a fan of SingleSetTraining routines, the only one that interests me is DC training because it takes a dual factor approach to periodization, which is far more fitting for guys who are already further along in their development.
You raise good questions though Trukk. How much volume is needed? And you are absolutely correct, soreness is a piss poor indicator of a workouts effectiveness.
If a guy is doing only 50 total reps of Back Squat a week, with a 10RM, and another guy is doing 100 total reps per week, I would say in most instances, assuming fatigue is being managed properly and both routines are properly programmed, that the guy doing 100 total reps per week is going to make faster progress. He will get stronger faster and his physique is going to reflect the new poundage's he is capable of pushing.
Is it going to be twice the amount of progress? No, obviously not, and at a certain point the return on investment starts to drop dramatically - like shown in this graph
Personally I believe lifters should take advantage of their capacity to do work, and continue to take advantage of it as their capacity to do work increases.