Ignoring ordinals and the fact that some sats are apparently more equal than others, there is a huge difference between serialization and a totally open ledger of transaction history. Banks can see entry and exit points, but Chainalysis can watch the actual flow from wallet to wallet and track the exact trajectory of each coin easily. This is a huge amount of data, which means you’re not only forever associated with everyone you ever transact with, you’re associated with everyone they transact with too, and that data never goes away. Even if you don’t care about Chainalysis being able to monitor you, if you have a savings account then sending someone Bitcoin from it means revealing to them exactly how much BTC you own, and lets them watch all of your on chain activity forever after.

This is not cash. Even accounting for serialization, I never have to spend a second thinking about the consequences to my financial privacy when I hand somebody $5. I will never know if my $5 bill has been used for drugs or strippers or crime, and I can rest assured that if I choose to use it for those things, or donate it to my church, nobody will ever be able to track it back to me 10 years from now.

This is true if you only transact anonymously and P2P, which means not using it as digital cash. If you gave that Bitcoin to your grandma, or to a business, you might unknowingly be giving them Bitcoin that was stolen in ransom attacks or used in DNM’s, and which would get their account frozen if/when they try and exchange it for fiat.

Cash must be memoryless to fulfill its function as cash. It must be highly fungible and liquid in order to permeate all areas of society and circulate between strip clubs and churches without ever developing a history or an identifying characteristic.

Purchasing anonymously and P2P protects your privacy, but it changes nothing about the fungibility of the Bitcoin itself. If you use anonymous P2P Bitcoin as cash, you could be putting others at risk who might naively deposit a ransomed BTC into Coinbase.