The topic is quite misleading. Many coins do not have any purpose or use case besides being a way for the devs to make money of "investors". Majority of cryotocurrencies in the market fall under this category and would not be around in a couple more years or shorter.
Btc is expensive so it can just act as reserve asset to back up stable coins like usdt.
Btc is more like limited supply sacred asset wich is perfect to be usdt backer.
Firstly, why would you want to back up a currency which is already (not really tho) backed by another currency?
Bitcoin has different usecases depending on the holder's preference, and the market price does not affect the usage of Bitcoin.