Before mixing came around, from my recollection, there was no need to mix or coinjoin. Privacy preserving techniques were created as a reaction to the human intervention in the system.

If governments didn't spy, monero might have not needed to be invented.

Like all good discussions, one starts out purely on a technical level, marveling at the proficiency of these great inventions. As it develops, we realize that things are the way they are because of human nature.

We can make bitcoin fungible, and private, but it needs special tools, some knowledge, and some additional cost. Monero does that by default. But bitcoin is the one that commands the higher dollar price at the moment, and that is what matters to the mainstream. Also, one must consider exactly what percentage of bitcoin has actually been used in a crime. How many of these coins are tainted? Apparently enough to make a monero maximalist drop their bitcoin. But this is the only example of bitcoin not being fungible, that has been presented to me. Unless there are other examples of bitcoin not being fungible.

A fiat currency example of this is are the Euro and Indian rupee. The Euro used to have a 500 note, and the 5000 Rupee note was recalled a few years ago by the government. Those notes were made useless. If the US government decides to eliminate the $100 note, it is the same idea. Government and humans take the fungibility away. If everyone used bitcoin on their own private wallet, even if a bitcoin was used in a crime, it is only on KYC type exchanges that is tracked.

This is why many folks will save in bitcoin, but spend in monero.