Could this have been a fatal typo?
M109 S2100; wait for temperature to be reached
Nah … firmwares will usually have a safety cutoff.
Good for testing the upper limit 
Only catastrophic if MaxTemp isn’t defined inside your firmware, I’m using Marlin. The setting is accessible inside Configuration.h.
Luckily I don’t think it’s easily possible to compile Marlin firmware (hopefully, all others too) without MaxTemp being defined. #ChecksAndBalances
Most firmwares have a maxtemp that will shut down if the heater reaches that temperature.
I’ll confirm that MAXTEMP works. I swapped hot ends, and was trying to heat to nylon temperature, but couldn’t get there. I had to change the MAXTEMP and reflash.
Still, a hardware safety would probably be a nice touch for our printers. Having serviced photocopiers for a few years, I note that the fusers had temp sensors and firmware had limits which is all most 3D printers have - but fusers also had thermal fuses in line with the power to the heater. A slight redesign of the hotend to accommodate these little button sized fire-preventers might be a good idea.