I can absolutely tell 65°C is too hot, that's 5°C short of what is piped as literal hot water in the taps in my area.

I would not recommend going above 40°C for washing, and there is literally zero issue remembering that. Body temperature of a healthy human is 36,6-36,7°C (97,9-98,1°F), everything above that is hot.

As such, there is literally zero issue figuring 40°C is reasonably hot and 65°C is unreasonably hot, it doesn't take a genius.

Speaking of water, Celsius is obviously superior as a water-based system. I can easily tell temperature in my kettle goes to 100°C (212°F, huh?), or steps down to 90°C (194°F??) or 80°C (176°F??) to brew a perfect cup of tea. When temperature outside goes 0°C (32°F??), I know I can expect ice and snow. And for everything in between, I can make a pretty accurate approximation.

And finally, modern Fahrenheit scale is literally defined through Celsius. It's a scale that is defined as 32°F at the freezing point of water (i.e. 0°C) and 212°F at water boiling point (i.e. 100°C). You're literally using Celsius but make it harder for no reason.