I am going to be realistic to be quite honest. I doubt we will ever see a 600k USD Bitcoin, well not in my lifetime anyway. The fact is people can make a crap ton of money shorting Bitcoin, so why would they even bother to try to pump it up? Only if there was a huge influx of people coming in to buy it in droves and holding it, would there be that kind of a chance, and even then the Powers that be could do everything in their power to manipulate the market and keep the price low. Look at the price of gold for example, it was held down for years and years. I think they will try to do their utmost to keep the price of Bitcoin low so that they can accumulate and own in the end most of it. This would technically make it less and less decentralised as bigger fish would own the majority of it like Rich people do with every asset class.
I find your "realistic" view quite pessimistic. Bitcoin has already shown that anything is possible. It grew from 1000$ to 20000$ in about a year, when some people thought that after the previous (1200$) ATH it cannot get much higher and people could make a ton of money with it already.
Indeed, some try to keep the price down in order to accumulate. But the competition for those coins is rising, since more wealthy people/institutions want a slice of the pie. So the price is bound to rise.
Your comparison with the gold is not good and I'll tell why: first, Bitcoin is scarcer than gold and second, with the advance of technology there can happen 2 things: banks can find something better than gold as store of value (it doesn't matter what!) and release tons after tons of gold onto the market - and - gold may be mined from other sources, not from earth ... all may sound SF but could make the gold price collapse. Such uncertainty can keep the price down (!).
Bitcoin decentralization is not about the number of whales. Bitcoin decentralization is about full nodes. As Legendary you should know this. So I'll not use that wording. And no matter how rich some whales will become, yes they will be able to manipulate the price for a while, but it will become more and more expensive for them to play this game.