Old school.... Is kind of what you said, if someone rides with their foot touching the clutch it will cause the clutch to get hot and glaze, which can lead to wearing the clutch pad and or warping the plate. leaving the clutch engaged is better for every thing, from panic stops, to standard stops. Practicing for panic stops is a good thing, and thinking ahead is always the best way to slow down what's going on around you while traveling down a road....... Try this on for size...... Look at the road as close to the front of your car as you can and still be able to keep it on the road, it looks like every thing is going by so fast it's making it hard to prepare for a good line though the turn(or to see someone cutting across an intersection with us being able to do something about it in time not to get hit by them.........
Now look down the road as far as you can and focus on the big picture, like when the road turns long before you get to it, it allows us to set up for the situation......... Now while looking down the road make a note to see how slow things are coming at you, it gives you a long time to travel though that space, and more time to smooth out the motions.
What you're doing IS planning ahead by stuffing it into 1st at speed, but practicing how to hit the brakes as hard as you can to activate the anti-skid and still leave the car in gear up until the last moment before the wheels stop rolling is going to get the car to slow down the fastest, the drag of the engine helps the brakes.
An exception: Back in the mid 70's on my mc, I was caught up in a panic stop where I grabbed the front and rear brake like we were taught....... I thought though it afterwards(to see where I could have wow'd it down faster), and the mistake I made was not pulling in the clutch at the last couple of sec., and it was the throttle hand that kept some of the gas on, which was helping to propel the mc forward, if I had let off the throttle "all the way" I would have saved some feet in getting her stopped. So I started practicing my hard stops with that in mind, and learned how to back off the throttle wih palm while using my fingers to pull the brake, easier said than done. But like
@webby, said these cars are programed to make the engine back off on it's own under hard stop situations. So to have us push the clutch in early negates why that feature has been installed.