Worth noting that Mac lets you remap the control, alt, and cmd keys to one another. It's been hidden in the keyboard settings under a "Modifier Keys..." button for a major version or three now, and is stored per keyboard (nice for mechanicals with a keycap swap).

The new MBPs (possibly anything on Sierra?) add CapsLock as a mapping target and both CapsLock and Esc as a mapped value, presumably to get around only having the soft Esc key in the touchbar. It's also nice for Esc as meta, though.

That obviates the need for Karabiner in the 99% case, though I still use Keyboard Maestro for macro and text expansion.