I'm not that good in financial thingies but I know that Bitcoin already have value even without fiat on it.

If you will dive back in the history, merchants are just writing on a paper and making an agreement that the paper is worth X amount of dollars. That is how money works in the past until there are developments. To answer that, Bitcoin still has a value as long as the 2 parties agree that it has value and the other party wants to accept the Bitcoin.

---

Both of them are important but in terms of usage, its obvious that fiat is the one being used by people still. Yes probably there are some who are accepting Bitcoin but if you will make a survey with this question to some random people, they will choose Fiat since it is the well-known and most used currency we are using right now.