Once there becomes enough sellers, selling a particular item though, for Monero directly, they wouldn't need to use USD as the unit of account because they could just compare their item with similar items being offered by other people to know what they should price their item at. In that case, they would have nothing to do with the USD price. Once you get enough of those items being priced in Monero, people will stop looking at the USD price because they will be able to see that an expensive car costs this many Monero while a cheap car costs this much less Monero, etc. And the USD price won't be a factor.

If UoA change happens it's not gradual over a time span of 15 years like with Bitcoin. It will be abrupt.