"Squatting to 80% 7 days a week is ridiculously easy - start with 3 max days to failure and 3 80% days (+1 deadlift day)"
I think you misunderstood something, the idea is to start by maxing out 3x/week and add a light day when you stall, then eventually make that a max day as well and later add another day and so on. Your suggestion could work as well, but in my opinion it's better to start with less and then add more rather than go all out and have to cut back. Mendes' backoff guidelines are reasonable, but what do you think of squatting to failure every day? Personally, I found maxing out on bench every day to work better than maxing out on squats, of course everyone is different.
Now about Mike Tuchscherer's idea of adapting to high volume, there is some truth to it. First of all, with his training system you do gradually increase volume over the longer term, just like Boris Sheiko would have you do. Ivan Abadjiev increased volume through frequency - you might train a particular lift 4-7x a week in the beginning, eventually you would have to do it multiple times a day to continue progress. However, there is a way around this. First of all, Greg Nuckols recommends using daily max squatting (with no down sets)and benching both as a peaking phase and to resensitize yourself to volume. Damien Pezzuti does volume/hypertrophy blocks here and there to continue making gains and add muscle mass. And finally if you look at Mike Israetel's new book, he advises using hypertrophy phases with higher reps to avoid accommodation to low reps/heavier weight that you would use in your regular strength phases, and vice versa.
The moral of the story is that at some point you will have to either change the stimulus or increase the volume to continue progress. It's true that overload in terms of weight is necessary to continue making gains, but work capacity increases you will have to increase either intensity or volume to see results. If you are already maxing out every day there is only one thing left to increase because you can't hit PR's every day. Sheiko has an interview on JTS, he says that you have to increase volume to a certain point, but once you can't increase volume anymore you have to increase intensity. This applies to any sort of training. You know the Polish weightlifting team maxes out on squats 17X a week?