Is it really all this simple?

Training stress = volume x frequency x intensity (or something like that). I have read a bunch of triathlon coaches describe their particular philosophy for applying training stress over a season. They disagree, sometimes violently, over which variable to change at which stage. But they seem to agree on this equation. They also agree on this second (sort of) equation:
Improvement = consistent and increasing training stress over a period of months with adequate recovery. It occurred to me while suffering on the trainer this morning that most of the disagreement among coaches is bunk. If you cannot plan a schedule that has you increasing the training stress consistently over a period of months, however you accomplish that increase, you are not going to get faster.

Put differently, the consensus seems to be that consistency is the key. If you can do high volumes week after week, month after month, then, great, you will improve. If you can't put in consistent, high and increasing volume, then you won't improve much unless you work with other variables. if volume and frequency aren't flexible, you can steadily increase intensity. What matters is finding a volume that you can sustain, and you have something to work with. I don't think it matters which variable you move, so long as you hold 2 constant and increase the 3rd. Arguing over which variable is most efficient to produce fitness gains is a nice intellectual exercise, but if the most efficient solution doesn't work in the your world to allow consistent training, it won't work for you.

Can you steadily add more workouts each week, or ride longer and longer every week for months on end? I can't. Can you disappear for 4-6 hours every Saturday, all year, and another 2-3 hours on Sunday? I can't. Can you disappear 75-90 minutes on Saturday morning and another hour or two on Sunday for 6 months and then start riding/running longer? That I think I can do.

Can effective training really be as simple as identifying which two variables you can hold constant and which one you can vary/increase? I don't know yet. But logically it makes sense.