on services offered, then its up to someone if he would use up his crypto to avail those service or buying up something or rather would opt out always with fiat and save up crypto for the future.
Its a matter of choice and these decisions will really differ for each person.
But as much as I love crypto,,, it is not possible to opt out of fiat at all. And I bet you very few people are able to do that. Even those who claim to live purely on crypto actually what they did is bought a house and a farm (thinking of that Dutch guy who sold everything for Bitcoin) so they are self-sustained. Still need fiat to pay bills!