IMO it has already started, actually quite a while ago. it's just not widely publicized or noticed bu wide public.

but perhaps it would be spread wider and increase after certain command will be given. things are developing according to some plan, whatever it is. present stage as I can understand - some sort of "controlled demolition" of cash fiat money, while at the same time gradual transition into "Cashless Society". they can't afford to keep present financial system to remain, but at the same time neither can abruptly "cancel" it, i.e. reformat too soon. thus it is some kind of a transitional period.

and part of it is "de-dollarization", which is in the full swing. which relates to this your question asked:

What happens to the value of the dollar when what we used to buy with fiat we use crypto only

first of all, dollar has no value. LOL :D
perhaps ever since it was stopped being backed by gold in early 90s.
it is a BIGGEST Ponzi scheme (or "scam" / fraud), along with all the other fiat currencies, but biggest of all of them - being the so called "global reserve currency".

and it is more correct to say that now rather its "real value", or the absence of it, would be made more open and obvious.

as for usage of Crypto instead of Fiat - that's not sure, frankly.
more likely, instead of USD and other fiat would be imposed some or other "Crypto-Fiat". I have also pondered on all such potential scenarios in my own recent blog post. IMO USD (and its clones) would simply "morph" into their Digital versions.

If that happens - then perhaps the whole Crypto would be effectively made off the limits / illegal, etc. Perhaps now it is already starting slowly and gradually - as provides example news article quoted here by you.