Protection of purchasing power?

There are various reasons people own Bitcoin but the main argument I see is a means protecting your purchasing power but I find that reasoning paradoxical in a sense. If you’re protecting the purchasing power of you’re fiat you’re essentially betting on someone paying you more fiat for some amount of Bitcoin. That just seems illogical to me. Surely to protect the purchasing power of your fiat you’d buy productive assets that produce more fiat? Am I thinking about this in the right way? I’d understand the logic if the world was denominated in Bitcoin and accumulation was the aim of the game.

Can someone change my mind? How is Bitcoin a sensible store of wealth?