So you’ve not answered the questions of “why would I spend this currency on _anything_, including other investments, if holding onto it is the best investment out there?” And how does bitcoin not consistently stay in a deflationary spiral?
Different people value different things differently, thus there is no universal agreement on what the “best investment” is. Remember that the price of Bitcoin cannot change without both a seller and a buyer. Each bitcoin bought is a bitcoin sold. The most obvious and simplistic example of a reason people might sell Bitcoins right now is that they are hungry and want food, but that generalizes to more complex differences between individuals’ time discount functions.
Keeping savings simple at times is prudent. I have a feeling a lot people are going to be mostly in cash in the next 18 months...
I also save my USD “as long as I possibly can,” by which I mean “until I value something more than the USD it costs to acquire.” In practice, that very frequently ends up being relatively small purchases of things I literally need to survive and function in society, like food, rent, and clothing, but beyond that I try to save as much USD as I can despite it being nearly guaranteed to decrease in value over time.