And no Fiat will be not worthless if that is going to happen but first you must let it happen and than we all can talk futher, the price is not even rise in the halving now.
Fiat is can not be worthless because you can not use bitcoin directly to buy you want you need it first to convert it and use fiat to buy things.
It's not really just in our dreams. Bitcoin used to seem impossible to reach $1 in the first years. Then it seemed impossible to reach $10, and then $100 and then $1000. Now it seems impossible for the price to reach $10k, but in my honest opinion.. With the recent halving, it's not impossible at all.
Which halving are you talking about? Halving has already done and now we have to wait for next 4 years and if you are expecting $10k price from next halving then that is impossible to achieve in such a short time frame.