It's the power button, just force instant power off if you don't mind an integrity check at next startup!

there is, sort of, but it's got one or two extra steps (and most folks probably don't know it exists).

1. Press Cmd+Option+Escape to bring up the Force Quit menu

2. Press Cmd+A to select all running apps

3. Click the "Force Quit" Button to force quit all apps

And then it doesn’t work, so after half an hour of staring at a grey screen, you give up and hold down the power button.

Why wait at all in that case and not just hold down the power button right away?