Bitcoin rivals are sometimes referred to as legal tender.
If you look at Bitcoin as a currency, you will need to take away its share as a payment method for the current fiat currency.
In that case, I feel that the biggest enemy is the US dollar ($), but I personally think that the biggest enemy is the euro (€).
The euro is a currency that various countries have begun to abandon their original currency. By doing so, it is attractive that the same currency can be used in other countries.
It is a currency that has increased the scale of the currency by increasing the places that can be used in this way.
This "increasing the number of places that can be used so that it can be used for payment in any country" is very similar to what Bitcoin is currently aiming for as a [currency].
Therefore, I think that the euro and the euro area are very afraid that "the euro will be abandoned and its share as a currency will be deprived".
I hear that Europe is quite strict with Bitcoin, but I think that is also related.
Therefore, I think that the biggest obstacle to Bitcoin's biggest rival is the euro, which has grown in the same way as a Bitcoin.
On the contrary, I think that the US dollar, which is the largest legal tender, will be able to coexist unexpectedly, even if it is not cooperative, including comparison of mutual values.
There are already a number of US dollar stablecoins.