People will still use or buy Bitcoin even though it is a stable coin. Have you not observed that USDT, BUSD, USDC are being used and traded because they have some usage? Besides not all who buy bitcoin are for profit. Other use it to buy merchandise because their banking system is too strict and give them limitations. Others even use it to transfer funds or use as remittances to provide help to their relatives that is on the other part of the globe.
Definitely, it is decentralized unlike those USDT, BUSD, USDC, and other stablecoins.
And people won't see a reason to buy Bitcoin if there were no Volatility from day one.
It isn't the volatility, instead, the volatility discourages lots of people from holding Bitcoin. It is the hope that one day Bitcoin will moon that encourages most buyers to hodl Bitcoin.