I understand where you are coming from but, again, it appears to me that you are conflating subjective perception of value (or user preference) with fungibility.
Not at all. I don't think you fully understand money. Subjective perception of value is all there is. Money only exists in the mind of those who value it. If people don't believe that two satoshis are equal, then they are not equal.
Therefore what we are concerned with is whether or not one satoshi performs the same function as any others. It seems to me that for all practical purpose they do and that Bitcoin makes no distinction between any of them.
Bitcoin does make a distinction between them by way of a public and browsable blockchain that shows you the history of any bitcoin since inception.
One bitcoin does not equal one bitcoin.
In fact, I would suggest an individual would be hard pressed to himself distinguish one satoshi from another. Consider this thought experiment:
....
I'm sorry but I seemingly cannot wrap my head around this "chasm" you speak of for the simple fact that as Bitcoin adoption grows it is inevitable that satoshis will cross paths with what you call "tainted" coin, so much that at one point one could probably suggest that a majority of bitcoins in circulation have come in contact with "tainted" coins.
http://sabr.io/ Blockchain analysis is already pretty good and only going to get better. A plea to the ignorance of the users is not a good argument.
The final nail in the coffin of bitcoin's fungibility *is* Satoshi's coins. If all bitcoins were equal and fungible, then it shouldn't matter that Satoshi began spending his coins. Upon receiving one you shouldn't panic that bitcoin users might start losing faith. But when dealing with *collectibles* (which bitcoin is) then that becomes an issue.
If you like fine art, and suddenly lots of famous paintings start winding up on your doorstep, you would reasonably assume that the popularity and faith in fine art is plummeting.
In Monero we don't have a Satoshi. Every user is free to transact, hold, and spend however they choose. The currency exists independent from the history of each unit. Gold and the dollar can be traced (with great effort as well) but they still pass the test for fungibility because without expending a massive amount of resources you can't effectively trace their history (especially with gold). With bitcoin tracing the history of a unit is trivial, and with Monero it's impossible.
You've basically strengthened my point:
Monero isn't just true digital gold. It's the equivalent of if gold automatically (and magically) went to a smelting facility every time it was spent and was melted down and mixed with other gold before coming back to the new owner.
It's the best form of money that civilization has ever seen.