I don't want this. I would rather push a button and wait for a light to turn on. Automatic off, fine, I guess, though I don't love it and would never want to rely on it. Automatic on, no way.
Awesome. Thank you!
How is that worse than it just being full temp the whole time?
And it's going to guess wrong a lot of the time. Automatically turning on and off both have unsafe failure modes that lead to it being on unexpectedly (it turns back on when I don't expect it to, and it doesn't turn off when I do expect it to) based on imperfect sensing hardware and software that can both stop working, and I'm not ok with unsafe failure modes in a device that will burn down my house.
This is also the reason that I disfavor battery-powered soldering irons in general, but at least being portable adds something you may need and can't otherwise achieve.
I've never had it guess wrong -- my hand isn't so steady that it will turn off while I'm using it. If it's on and on the table, it is still visibly ON so this is just extra safety if it's a little bit cooler.
Other times you just want the equivalent of a drill or toaster. Pull trigger, drill spins. Twist chuck or shift gearbox, it slips or changes speeds. Push toast down, it toasts, twist the dial if you want darker or lighter.
An on/off switch, a potentiometer or 7-segment and some buttons to set temp, and a nice, fast, powerful PID loop to control the temperature (with a 120V AC cable to make 100W all day not a problem) is all I want in a soldering iron. I have a combination soldering/hot air station that's almost 20 years old, it just always works.