Of course there are many advantages of bitcoin over fiat, but it has a different purpose to buy bitcoin. Some people only use it as a payment alternative, while others make it an investment and trading asset. Simply put bitcoin is a great currency or investment asset, not just fiat and other investment assets where decentralization is one of the things that makes user feel like he has bank in his hands.
You are correct that Bitcoin has several advantages over other traditional fiat currencies and one of them being cross border transaction fee which is much lower compared to using traditional banking channels for this purpose. In my view the key advantage of investing in Bitcoin and holding it for long term, is its unmatched returns which is evident from its track record of past 15 years since its inception, despite the fact that it is highly volatile asset.