I have been pondering on the motion of making bitcoin a legal tender, if bitcoin becomes legal tender, it can be used in place of fait currency to purchase goods, used for transactions,etc.
No one wants to replace the fiat currencies. Moreover no one also wants Bitcoins to be treated as a currency.
I agree Bitcoin is not created to replace fiat currency but the creator Satoshi Nakamoto thinks of Bitcoin use case as a currency (a reason why he noted to call Bitcoin a cryptocurrency) or mode of payment...
Here is some part of an article stating about Satoshi's discussion of Bitcoin use case with Malmi:
Any use cases?
Satoshi and Malmi frequently discussed potential uses of Bitcoin, knowing that the network needed a killer use case or application to drive adoption. Generally speaking, Satoshi noted the timestamping features of the blockchain, which could be used to help authenticate events in the real world.
But Satoshi also thought about bitcoin’s position within the world of digital payments, and thought it could be used to create more liquid markets around existing digital currencies like Liberty Reserve (now defunct). He predicted that people wanting to improve their privacy could go from bitcoin to liberty reserve to dollars, gold or PayPal, given that at the time it was easy to generate BTC just by mining.
One avenue that Satoshi correctly predicted would be bitcoin’s viability for buying gift cards (which he referred to as “paysafecards”), which today is one of the most common uses of bitcoin.
I am also treating bitcoin as a currency since I am using it to pay for my online purchases. Many maybe thinks Bitcoin as a digital asset but we cannot deny the fact that Bitcoin was first created as "A purely peer-to-peer version of electronic cash that would allow online payments"[1] So even before anyone thinks Bitcoin as digital asset, it is first treated as a currency.
[1]
https://bitcoin.org/en/bitcoin-paper (Bitcoin whitepaper)