Bitcoin is a perfectly closed system. Every 10 mins a new block and so it operates like a clock.
The value it captures is dependent on the amount of liquidity in the system. As liquidity expands it captures more monetary value and as liquidity contracts the monetary value goes down.
The amount of BTC in the system does not change outside of its fixed issuance rate which is deflationary. A deflationary currency will always out perform an inflationary one due to its scarcity/hardness. Hence gold outcompeted silver as money over time.
Because we know that fiat currencies will remain inflationary and governments must increase their money supply to remain solvent on their debt we can safely assume that as long as Bitcoin remains decentralised and secure and M2 money supply keeps growing the dollar value and hence purchasing power of BTC will continue to increase over the long term.