If Coinbase was the processor of BTC, then I would agree with you. But Coinbase is not.
The P2P and mining network is the processor of BTC. The P2P and mining network will always consider BTC to be perfectly interchangeable, 1 BTC is always equal to 1 BTC regardless of history, law or anything else. The P2P and mining network will always consider 1 whitelist BTC to be of equal value to 1 blacklist BTC, and process transactions accordingly. This maintains perfect fungibility.
If you argument is that entities (goverments) will try to make individuals value certain coins (whitelist) more over other coins (blacklist). However if they can dictate usage of bitcoin, they can dictate usage of anything, including Monero. Either way it does not matter though, to the P2P network BTC are all still identical, this is fungibility.
If Bitcoin was not perfectly fungible, then you'd have a situation were the P2P network and the blockchain processes 1 blacklist BTC as somehow equaling less than 1 whitelist BTC. That is impossible with the current network and would require a hard fork.
Well I disagree with your definition, as I explained earlier. Fungibility exists within a social context. If nobody cares about the history of Bitcoins, then they're fungible, but if people do care, they aren't. I have some Bitcoins that were mined by me. I'm not interested in trading them for Bitcoins that for all I know might have been stolen from bitstamp and blacklisted by btc-e or others, or might otherwise cause me problems in the future. Not even for a small premium. That's not fungibility by the accurate definition.
that's your choice. but i expect that you continue to accept USD bills routinely despite them being covered in drug taint.
likewise if most of the actors in the Bitcoin system don't care or more importantly understand that tracking tainted addresses makes no sense in light of the fact that UTXO's are what gets joined and split in essentially non-trackable ways then what you decide personally to do doesn't matter.
and this is what i see as happening; at least for the last 6 yrs.