There's no fundamental link between the cost of storing data on a blockchain and time. However, practically, it does look like the most common blockchains have

There's no fundamental link between the cost of storing data on a blockchain and time. However, practically, it does look like the most common blockchains have gotten more expensive over time: Bitcoin, Ethereum, even Dogecoin. However, you could imagine a blockchain that gets cheaper over time i.e. by issuing more coins or even stays the same.