Like I mentioned earlier, I actually do not believe that all countries in the world will accept Bitcoin or cryptocurrencies at large. But the post if about ‘what if’. So the responses are tied to the assumption of Bitcoin being accepted by every country in the world. And like you agreed, $100k as the value of Bitcoin is small as it will likely be way more than that considering that it isn’t just a couple of countries or the top countries but every country including all big countries too. That would be massive ‘if’ it were to happen.
If this had to be equalized for all countries in terms of accepting Bitcoin like accepting other currencies in life, of course the threat would be the value of the fiat currency in each country itself. Even though Bitcoin does not aim to kill any country's currency, this could happen as a result of Bitcoin's increasing use and increasing trust by many people so that there is a possibility of experiencing major changes in its value or price. So I wouldn't assume it's the same for every country in their desire to accept Bitcoin (if that actually happens)