"I suspect magnetrons are the same way. "

It seems CRTs are. I had a Zenith 20" come in, had the best picture I ever saw on that model because the tube was strong. I mean STRONG. Te complaint was there was a slight noise when it was turned off. I discovered that it really wasn't turning off, when the micro said power off it muted the audio and video but the HV, and of course the filament of the CRT and all that was still on. I told them not to fix it. Management didn't agree so it got fixed.

We knew for a long time about CRTs, like NEC bigscreens had active all the time AKB and when you first turned them on they were actually "cathode stripping". Sony's AKB had a delay until beam current was normalized, as told by the AKB circuit. Didn't help, their CRTs were junk by then. But I got this 35 year old Sony in my room with about the best god damn picture you ever saw, and it is never shut off. To shut it up I just switch to a dead source. You will NOT see color like on this thing.

Anyway, the magnetron has a filament so when it cycles on and off the filament thermally cycles and in almost ANYTHING in the physical world thermal cycling is bad.

The old microwaves with the tap, the filament voltage was still the same.

Actually if you look at the power level switch on one of them you can see it is made to switch a few thousand volts. It is not a simple slide switch.