There will always be poor, rich and ultra rich people no matter what currency are we going to use. However decentralization in Bitcoin is about not being controlled by a central entity. For example the Feds can increase the supply of US Dollar indefinitely. Also they control how much liquidiry is poured into the market, etc.
Well, you are partially countering your own argument.
If I have lots of Bitcoins (say, 5-10K), I can easily control the liquidity in the market. I can easily fake pump/dump by choosing any single exchange and that will manipulate the whole market.
That's the current misery with Bitcoin. Although the Proof of Work algorithm ensures its fair-working as a currency (since lots of nodes already ensuring decentralization of chain's instance) but 'Bitcoin as an investment' is still prey to the early holders. That's what make Bitcoin a risky investment and main reason why it is referred as Bubble by many experienced investors.
But it doesn't work that way, because the moment you try, you will find your operations don't execute instantly for the entire amount. The capacity for whale manipulation is lower than some people think. This is why this "misery" doesn't even exist.
Wealth distribution is unrelated to coin centralization, that is a fact. No one can destroy Bitcoin overnight, like governments, banks, wars, etc can with fiat. Those that invested earlier, profit the most, and there is no turning back. Bitcoin is no longer something to make money by just buying and holding, that stage is already gone. It is a coin that keeps its value over time, that's its intended purpose, and thanks to its design, its smoothly entering this stage.
Before Bitcoin the world did not have this kind of coin, now it does. Even if you (like me) missed the early days, we can still use it to at least preserve what little wealth we may have up to this point. Should you keep it in fiat, you risk losing it overnight.
Suppose a catastrophe (natural or artificial) happens in your country tomorrow, if you had moved all your money into Bitcoin, your wealth would have been kept safe even if everyone else around you enters misery due to the fiat/economy collapse. But this is a decision you can only take BEFORE it happens (even if it never happens). Once it happens, it is too late to react. The smart investor would simply diversify, just in case. It is foolish to trust your government or your central bank, especially when you know they have the power to make you poor overnight.