The short answer is, fiat currencies are purposefully inflationary
The long answer is that Bitcoin has been made deflationary, due to a hard cap, coin loss, hoarding, or whatever. In the view of Gresham's Law, Bitcoin is good money to be stashed away, while fiat is bad money to be spent shortly. So whenever there is a choice to be made between what to spend and what to save, the choice is evident and straightforward, so there is no need to name it here
That's correct, except that we cannot apply Gresham's Law to Bitcoin. It only works with commodity money and only given that the price of both good and bad money is set exogenously. In our situation Thiers' law is more appropriate, which states the opposite to Gresham's Law.