Charlie, to some extent I will agree with you and what you wrote. HST is really the ticket for mowing quickly compared to gear, and you can keep the tractor from bogging down by slowing down.
HOWEVER, the one place I will depart is that we all have our own turf conditions, and that said, someone with thick turf may end up driving so slow in areas that they could actually mow faster with a smaller (more correctly sized) mower deck. The square footage arguement you state makes some sense, but a smaller sized deck will have less initial blade tip contact and therefore will be able to run faster through more of the lawn (or all of the lawn) and while the width may be narrower, the net effect may actually be less overall cutting time, and far more importantly virutally nill possibility of bogging down the engine and causing engine damage. Further, a properly sized deck will require less basic hp to run becuase it is running slightly less mass and the tractor is hauling less mass as well, so more useable hp is available to the blade tips of the properly sized deck which allows the tip speed to be kept up to optimal speed with less load on the engine.
I have areas of my property where the lawn is reclaimed pasture, the soil is clay and even when it gets tall, it never gets thick. No chance of bogging my tractors down on that area. I have other areas where the grass is very thick, lush and mixed with clover, I bog down my 60" deck (19pto hp) if I let it get too long (about 6 or 7").
Properly sizing ANY implement is important to the longevity of the engine. Using your arguement, which again I agree with IN THEORY but not in practice, I could easily hook up a 10' Bat Wing mower deck and simply run the tractor in creeper speed. I can't say that would be a practical thing to do.