Exactly one of bitcoins most valuable features is its scarcity. The fact that only 21 million will ever be made..if you think about how many people are in this world, it just goes to show how truly scarce it is versus so many other coins (or even the US dollar).  All these coins with unlimited supplies are just silly.

Why bitcoin is valuable?
Bitcoin is valuable because it as a limited supply and there is a demand for it. There is a demand because people can use bitcoin as a means of exchange( an act of giving one thing and receiving another).

I agree with you, Bitcoin does have value because there is trust from investors in Bitcoin. This means that with supply and demand it will make
Bitcoin valuable, imagine if there were no investors who wanted to buy Bitcoin, then Bitcoin has no value at all. Therefore it is very important
Bitcoin has the trust of Investors. Bitcoin is highly trusted because it is different from fiat which is controlled by the government. Bitcoin is
a decentralized currency, we can carry out financial transactions without the need to go through a third party. Then the very limited number of
Bitcoins also makes Bitcoin more valuable, in contrast to fiat which can be printed at any time by the government. That's why the popularity and
demand for Bitcoin continues to increase, because Bitcoin provides benefits that fiat cannot provide.

It's a pity that many exchange, project are all going centralized, making that anonymous, private and even security nature rotten away, now some exchanges is making life difficult for it's users, Bitcoin would always be valuable but if we keep making crypto lose it's decentralized nature then we may just be having alot on our hands

Bitcoin is valuable because it has properties that make other assets valuable: scarcity, acceptation....
While having other properties which not all other assets got: Easy to conceal/store, divisible, borderless, censorship-proof, etc.

Let say that all 21 million bitcoin are mined, I still do not think that 21 million people can buy 1 bitcoin each because when the time comes when the 21 million bitcoin have all been mined, their value is also going to be very expensive and yes the op is right when he say that people cannot buy anymore after owning 1 bitcoin, because they will now be broke or full of debts after that lol but seriously if what the op means is about the circulation or obtaining a new bitcoin then its still possible because like you said most of us here that involved in bitcoin are only for the profit, we do not hold bitcoin forever so the bitcoin still circulates from one person to the other.

Simple, Bitcoin becomes valuable because people are willing to exchange it for real goods or services, and even willing to exchange it for cash. that is the point of a valuable and rare asset, it's just a matter of time bitcoin will be a difficult asset to get in the future

To be honest, I also got the same feeling when OP phrased his title as I was expecting it to be a question, but it turned out that it was some valuable information instead.

But like you mentioned, the reason on why BTC is considered to be valuable is due to its inflationary value and limited supply on the market. Having only a 21 million in supply means that its price is bound my inflation, and supply-demand. Considering that BTC is used as a form of currency that deletes the use of a third-party in handling our transactions, people can safely transact on the BTC network money from one person to another, without worrying too much on your identity.

For every people buying and holding a single bitcoin, the price will keep going up to a point that it will be almost impossible to buy a whole bitcoin with their current wealth. And with the price increasing, others that have bought will want to sell theirs to take their profit. So there will still be more bitcoins for people to buy. What would they do with the bitcoin if they never sell or trade it for goods? There would be no use holding it.