My claim is that anyone that uses Bitcoin as it is meant to be should absolutely not be concerned over fungibility.
This is incorrect. Any counterparty to a transaction can define things any way they like. If someone doesn't like your coins they can choose not to deal with you, or charge you more. It doesn't take a third party to impose such a view. It is purely a social convention how much weight to attach to to the history. If it becomes widespread, then it won't matter what third parties say, because people won't want coins with problematic histories. If history is widely ignored, then obviously it won't matter.
The important point is that nothing about the technology forces people to behave in one manner or the other.Moreover it can certainly be argued that these occurrences are marginal and nothing more than distractions.
I guess that depends whether you are the one dealing with the "complications" related to people paying attention to coin histories. I know several such people personally. It isn't marginal to them.
I'm curious who it is these people you refer to are dealing with?
Fiat institutions? That would make sense. Else I can't see how there'd be any important amount of individuals out there carefully examining the provenance of the coins and checking them against any blacklists or what not. I'm not saying it doesn't happen but that seems like a somewhat far-fetched scenario.
I, for one, could not imagine me refusing to business to someone on the basis of the history of the coin he is attempting to pay me with. On a long enough timeline, that can't be good for.. business. More resourceful individuals will show up on the marketplace that won't be bothered by such folklore.