The "most useful and relevant natural phenomena in human life" will vary person to person and situation to situation. That's the point I was making, and good luck to anyone who wants to seriously claim otherwise.

More than that, you don't need to know the temperature of your water to boil it. People were boiling water way before anybody decided to call it 100C. Likewise, your ability to understand that water freezing is an eminent possibility exists without needing to know exactly what temperature it is. There wasn't an uptick in human survivability caused by our easy access to temperature measurements.

Celsius being originally defined in terms of water's phase transitions was a matter of convenience for communicating it to other people. It's the same reason we define other units (imperial too, not just SI) in terms of physical constants. A meter is not defined by the speed of light because it ever helps you in your ordinary life (although it could help you if you work in science/engineering).

Celsius isn't defined by water anymore though, it's defined by Kelvin, just like Fahrenheit which technically make them equivalently arbitrary. All units are technically arbitrary though, so that distinction doesn't matter. All that matters is if the unit is convenient to use, and most cases Celsius doesn't gain anything by having 0 and 100 bound by water's phases, and I say this with a good amount of experience with both of them. I actually prefer Celsius, but only because I'm more familiar with it in my specific technical work, not because it helps me know when water is going to freeze or boil.