I dont think this has anything to do with fungibily.
1 btc = 1 btc as long as both are in the same time.

All btc In 2018 have the same value, which is not the same as that same btc (or any other) in 2021.

Usd is also fungible,
 1 usd 1830 = 1 usd 2021?
This makes no sense at all.

Maybe you live in a decent country where inflation rate is controlled and you may never had the opportunity to think about it.

But people from developing countries understand this very well.  Money value varies over time.
In brazil we lived more than 100% inflation per month. Interest rates are real here.

1 brl today is not the same as 1 brl tomorrow.  Eventhough Everyone will accept any brl today and tomorrow,  the same coin is different over time.

This is not about fungibility