Why does something have to become an international currency to succeed? Monero (or a crypto with similar properties - privacy, anonymity, strong fungibility) will most likely dominate black markets (AKA the only markets that are permissionless) especially with cash being phased out and the world becoming increasingly digital. The wider "shadow" markets are about ~25% of global economy and upwards of 2/3rds of activity in the global south.
Bitcoin might work as a reserve currency between enemy nations, but very unlikely that they will let their respective citizens use it unrestricted/unmonitored on their white markets. They're not going to start playing fair all of a sudden with a better money. Being valuable is not enough. How many businesses that we transact with everyday accept gold? Self custody will likely be outlawed or at least made unworkable as a currency with burdensome regulations and taxes in major nations. Just think of how you are automatically deemed suspicious or guilty-until-proven-innocent for merely owning large amounts of cash.
But yes fees, privacy, and fungibility are a few other reasons that I can't see it working as a currency (L2s have their own problems...). It's more akin to a digital asset/commodity.