But then recently an argument came to my mind which I think is the best argument against Bitcoin ever and nobody ever came up with it before me.
Bitcoin will fail because it is deflationary.
"What the hell??" you may be asking yourself. "That is Bitcoin's biggest advantage!".
Nope... The problem is this: The rich and really powerful people NEED a system that is inflationary because that will continuously increase their share of that system. And that is how it works with the US Dollar. As new US Dollar is printed the Dollar loses value which on first glance makes it look like an unattractive system because you are continuously losing your share of the system. But the truth is that the more money you have the easier you can get more of it and thus as more money is printed the share of the rich people increases because your share decreases faster than theirs.
So if the Bitcoin is unattractive to the elite - and it is - then why would they adopt it? Why would big corporations adopt a system that was designed for "us, the people"? Why would they invest into a system that is already mostly owned by somebody other than themselves. Nobody who has power has an interest in Bitcoin. And why should simple people like you and me win against the powerful elite?
Imagine you were a Bitcoin whale and a different coin became big. Would you immediately invest in that other system? No! You want YOUR coins to be valuable. Not the coins of another system in which you have no weight. And the same applies to the elite. They do not want to move into a system that does not favor them over everyone else when they are currently invested in a system that does.
Sure Bitcoin will exist and it will continue to work as a store of value for people like us. But it will never be adopted by the economy and it will never reach prices such as $10 million per Bitcoin. For this to happen Bitcoin would need to be used by everybody.
Building on what you have said there is another element that will actually sap away the power of bitcoin over time. Let's say that there is a fixed amount of 21 million bitcoin, the expected maximum to ever be created. Over time dust is left in various wallets, people die without anyone knowing they held bitcoin amounts and all sorts of mishaps occur that results in chunks of that 21 million bitcoin never becoming available again. Good for the bitcoin price getting pushed higher short term but bad for the actual usability as a currency as availability is constantly shrinking. Next you have billionaires buying up large amounts simply because they can and they have so much money being created around them that sinking it into a fixed access has insignificant impact on their future finances. Again, the pool of funds for everyone else shrinks, these megarich have no need to sell. Next you have companies like Paypal coming in and buying up huge volumes, but then locking it away internally without any ability to transact directly with external wallets - it's trapped inside their ecosystem. All of this is forever sapping away how useful Bitcoin will be in the future because the denominations get smaller and smaller. Who wants 0.01 BTC when you can have 10,000,000 of RandomCoin that has better prospects for actual functional and commercial usage?