Just think about it: if I can use fiat currencies that constantly lose value of Bitcoin that it's the best at keeping it, which one am I going to use?
Assuming the day comes sometime in the future that people worldwide can and have chosen be paid with bitcoin for their wages/sales— would you prefer selling the bitcoin for fiat
then spend it? Or just straight off spend the bitcoin instead?
Well, I think if market cap increases, Bitcoin becomes a unit of account and people worldwide can be paid with it, there won't be that option. Fiat will have disappeared by then, or it won't be long.
As long as there is coexistence of a crappy currency and one that is worth a lot, people will spend the crappy currency. And if there comes a time when the shitty currencies lose so much value that they collapse, the only option will be to spend Bitcoin.
Although this is very hypothetical about the future and we don't know if by then a decent cryptocurrency may have appeared that as good as a store of value as bitcoin and is better for everyday payments.