I respect what you're saying. And in addition, based on my experience, I respectfully suggest there is more to the picture.

First, I realize I was unclear: it's touching the edge that I was talking about, right as it's coming off the belt. You're absolutely right that if the mass of the blade is hot, the temper has already been messed up -- and the operator is a hack.

I'm not sure that all factory blades are messed up. That hasn't been my experience anyway. Mid-range or better blades usually "bite" properly on contact with hand stones (usually diamond for me) and that indicates that the steel at the edge is effectively the same as the steel in the mass of the blade. It's correct to note, though, that many factory edges are only fair-to-middling at best, and I find there is significant room for improvement.

Some lower-end blades, though, are absolutely temper burned right from the factory. Sometimes it's accidental, but sometimes it's a way of sexing up shoddy steel. I noted this in low-end Gerbers and early "Henckels International" blades out of China. It took me a long time to figure out WTH was going on. It seems that the layer under the extra-hard "burned" layer is tough, grainy, and highly abrasion resistant -- and it doesn't take an edge worth beans. Working through this by hand, to the "meat" of the blade, takes forever (with vigorous cursing). Only a belt sander or wet wheel can peel through it with any reasonable efficiency, after which the steel handles normally (for better or worse).

I still prefer hand sharpening, and especially hand finishing for good quality blades. There's a feedback from the blade that I don't get from the sander. But I think the scratch pattern from the sander is so much more consistent that, with practice, it might give a more durable edge -- respecting the temper, of course.