But most people’s monitors don’t, or people just never set them outside the standard rate.

My monitor's standard and actual rate is 165 Hz, which is probably not terrible at approximating 70 Hz (very few fps from a small multiple of 70 would be worst).

High fps is probably a little more rare than HiDPI, but also becoming more common. It's already common in phones.

Sure. I don't think I've ever had an LCD monitor that supports 70hz at a useful resolution though. My current monitors (native resolution 2560x1440) only seem to support it at 1024x768!

Well, the demo runs at a lower res than 1024x768 so you're all set ;)

I would just run it on my 386 with real CRT anyway haha

Pretty much every modern fast gaming LCD will support 70Hz.