Wifi turns off and refuses to turn back on on MacBook Pro Retina with OS X El Capitan
My AirPort Wireless card was broken

Turns out a spider crawled into my computer, nested itself under the wireless card, and died in direct contact with the wireless card. Go figure. The guy who looked at it said there was water damage (ew), and, after disabling the wireless card (software-wise), my crashes stopped as well.

The crashes were the main reason for him to suspect that the wireless card had issues, because the kernel was throwing fatal errors all over the place. If this happens to anyone else (and you know it's the wireless card), what you can do is the following:


Open Terminal.
Use ifconfig to turn the wifi interface to down (should be en0, so using sudo ifconfig en0 down should work)


This makes the computer stop querying the faulty device.

Go to Network in System Preferences.
Remove the Wi-Fi interface (hit the minus button while highlighting it in the leftmost column).


This made my computer stop occasionally crashing at boot.



I know that it was throwing fatal errors because, even though they weren't visible in the kernel logs, when I booted in safe mode, it threw about 50 errors in a second all relating to AirPort Wireless from ARPI.

I'm working on replacing the card - I hope this helps someone else.