I'm wondering what if one day Bitcoin goes so high up in value that it causes somewhat a fiat crisis? I mean imagine, you've Bitcoins but there isn't much cash available in the market to buy your BTC, or simply put people can't afford it? Will bitcoin be of any value then?
You mean like it affects USD price in a way that 1 USD would be worth less than 1 USD?
That's not an impossible scenario but it's very unlikely to happen because governments are going to intervene and remove the extra cash that is floating around to keep the current value around the same.
If cryptocurrencies or bitcoin gains a huge popularity, like a time comes than 90% of the population of world are using crypto instead of Fiat then it really could lead to a crysis for Fiat but not price wise though but it just devalues the use cases of Fiat and eventually in next 50-60 years they have to remove Fiat completely, but you can be sure that before it gets to that point governments make their own crypto and with new tax laws and coming up with new restrictions will make sure everyone use their own crypto, so they will try really hard to delay the removal of Fiat which would eventually be replaced by one or multiple true centralized cryptocurrencies.