However, the fact is that there are still many countries that prohibit bitcoin. In my country, bitcoin can be owned as an asset but not as currency. So anyway, without legality from the government, Bitcoin will remain in a gray area

It seems that currently Bitcoin is still in a "gray" area, even though countries ban it, in fact they are not serious about banning it because the government does not carry out raids or confiscate Bitcoin. It seems like the government is doing this on purpose, because if all countries agree to ban and eliminate the circulation of bitcoin then at least the country can mass close all crypto exchanges.

In my opinion, if bitcoin is legalized as a currency in all countries it will not shift the position of USD because bitcoin has a limited amount and there will be problems with transaction volume as you said, but at least it can be used as an alternative currency in cross-border trade