Even If the btc price will be 500k than many holders might sell some of the coins but always will buy when lower price and will hold some bitcoins.
No matter how high the price we weight btc value just with fiat currencies but it's deflationary against many other things like also to gold and If something is less then other things then off course it will have higher value.
The limited supply idea of something it's genius and good for long term holding to have something wich keep wealth safe.
Off course we can say that there is a lot of things less than other things weighted against them to compare their value but for example only btc now have the guaranteed mechanism of limited Supply and Halving.
So the btc are for those who want to be rich and stay rich.
These two concepts play around with each other and is the main reason why bitcoin's so valuable these days, it's technology is dated, a lot of other cryptocurrencies in the market does what it can do, but much better, and it doesn't have that much going for itself security-wise since a lot of governments have found ways to circumvent the anonymity aspect of crypto, but what it's great at, is being a coin.