You keep making these factually incorrect statements. My USD buys almost exactly as many AUD, yen, CAD and other major currencies as it did 25-30 years ago. There has been no significant relative decline.
I just calculated the relative decline of the USD against the AUD since 1996. It has declined at an annual rate of about 0.1%. It has appreciated against the yen in that time period.
Bitcoin has declined 12% against the USD in the last month for comparison.
I'll use some concepts to address this.
Image you have 3 cars, all travelling backwards on the motorway. (The cars represent currencies, and the road represents purchasing power).
Imagine the one going backwards at 5mph, a second going backwards at 10mph, and the 3rd going backwards at 20mph.
They are all going backwards (declining), but from a relativity perspective of currencies 2 and 3, t
he 1st car actually seems to be going forwards (or at least represents the best option in any attempt to go forwards).
This concept can be confusing for some, and indeed that confusion can be visually demonstrated. We probably have all experienced the phenomenon of feeling of our car going backwards when we see a car next to us going forwards. (Indeed this concept to some extent forms part of Einstein's famous "Theory of Relativity").
Now imagine a car actually going forwards. That car looks unusual, shiny, had strange jerky motions as it accelerates, but goes damn fast. (In this example, this car of course represents BTC).
For currencies 2 and 3, the BTC car now represents a far more attractive option. People used to holding currency 1 as the best option, now may feel uncomfortable / confused / disbelief with a currency actually going forwards in purchasing power (as opposed to being the "least slow" in going backwards), but it is what it is. And clearly if you want to go forwards (as opposed to being the least slowest going backwards), you jump on board the new BTC car. At first maybe only a few people notice and/or board the new car (as with anything new) and then over time, as confidence, interest, awareness and acceptance builds, more jump on board.
Or perhaps another example. Image you have 3 pieces of fruit. A slightly bruised apple, a very over-ripe banana with numerous brown patches, and then an entirely moldy strawberry. They are all to some extent rotten, but we would likely choose the apple of the 3 if we had to eat. At least we would until a beautiful pristine perfectly ripe and unblemished peach came along and was presented to us as an option. In this example, you guess what the peach represents.