You tread wear should be fine. Your cost of "nothing" is about 30k of extra wear on the rotated rear tires. Change is difficult. Much like those who think going outside in the winter without a coat causes one to catch a cold. The entire idea of getting great gas mileage is lower owner costs. So too is maximizing tire wear. Nothing personal, I'm just the 35+ year electronics/computer guy that everyone with a "google degree" likes to argue with when it comes to technology. I'm quite used to it. I was designing and programming PCL circuits with a 16 key pad when a middle school kid can do it now with a laptop but has no clue as to how to what he is actually doing.
The rears tires have another very important function -- to make sure the rear stays in the rear. For many of us ordinary drivers (not race or rally drivers), large traction asymmetry makes car handling less predictable on snow and ice and serious water. This was a big deal for those of us who learned winter driving before ABS was available. Tire rotation helped keep this asymmetry small. When rotations are combined with seasonal tire swaps, the cost is zero.
That will be offset by reduced wear on the front tires. Over the overall tire replacement cycle, it should work out to a net-zero.
Who in the heck pays $20 to have their tires rotated? That aside, I'm trying to figure this out because it seems to make some sense, unless I'm missing something, besides maybe some degree of safety by not having the best tires on the front. I get the point, you paid for 2 tires to get to the 84,000 mile point and you'll pay for 8 tires to get to the 168,000 mile point, 2 for the next 84,000 (or however long the new tires last), and so on. What I don't know is how many miles you might have gotten had you rotated or how many miles were still left on the rear tires when you finally replaced them? If my math is correct, you'd have needed to get 56,000 on 2 sets of 4 and that is highly doubtful. Even if the OEM tires had lasted to 56,000, the rears would have then lasted more than the 84,000, so the theory would still hold.
And an equivalent amount of less wear on the other two tyres. I'm not convinced by your argument so far as tread wear goes, and I'm very skeptical that having two old rear tyres out of four is a smart safety move when it comes time to rely on good 4-wheel brakes. In any case, by following Toyota recommendations for rotations and tyre manufacturer recommendations regarding pressures my experience has been excellent in terms of wear, longevity, and tyre performance. What is not to like ?
I question that trip computer sometimes. When I had my Prii, my trip computer never matches my calculated fuel ups. It's always higher by at least 10 mpg.
