But if bitcoin loses its decentralized nature, it will cease to be an investment and store of value. Bitcoin would not be an investment if it were centralized because if someone could control it, they could increase the supply and render it useless just like fiat.
Bitcoin has always been a thorn in the government's side, and if it becomes centralized, the government will control everything and even destroy it. So it can be said that the decentralized nature has made bitcoin successful today, if bitcoin becomes centralized, it will be useless, and the game will be over. The decentralized nature is at the core of what makes bitcoin different and unique.
The decentralized nature of Bitcoin is a concrete feature but we must admit that that is not the only reason why Bitcoin has thrived.
Fiat is not useless. It's still a store of value. Majority of the world still use Fiat. Less than 5% of the world population own crypto. So how many of these will be for Bitcoin? Besides, a good number of those using crypto still use fiat. And the over 95% of people in the world use Fiat as a store of value.
If Bitcoin was just a decentralized currency and it cannot be invested then I doubt it would as successful as it is today.
So it's right to say the decentralized nature and the investment nature are the things that make Bitcoin successful.