You are likely correct that there is a sliding scale when it comes to fungibility, and I think that most free market types are going to perceive the most value in bitcoin being associated with greater levels of fungibility... so yeah, you are right that lesser fungibility may maintain some value, but the amount of decrease in value may be a lot greater than what you seem to be projecting it to be. For example if confidence is lost because coins get blacklisted, then surely that seems problematic to me if it is allowed to occur. So if any 3rd party such as coinbase or fed government tries to label coins, then there would likely be some effort by the bitcoin community to either not use their services or to move coins to other location and clean them of their blacklisting.
I think there is an argument to be made that zero fungibility may cause an increase in value as governments would accept and regulate such a state. I can see a scenario where your
address will be issued and monitored. If this becomes a future scenario then TPTB would actually cause an increase in the value as institutional investment would increase exponentially. and Ironically BTC would become the exact opposite of what it was intended yet as a side effect continue to make early adopters rich.
I think that when a few of us speculate that there are going to be fungibility battles in the future, these are the kinds of battles that we contemplate to be within the realm of possibilities. Part of the problem about speculating about the future is that no one is really going to know how the battles might play out. I am fairly certain that varying levels of preparation for fungibility battles have been ongoing in bitcoin since it's inceptions, and even during my about 4.5 years in bitcoin, I have seen various kinds of fungibility discussions, even from core developers. So the topic is not lost on them and the tools for defending bitcoin from these kinds of challenges is not new, either. So, it remains somewhat speculative if fungibility challenges become more of an issue or a BIGGER problem that might need more tools - but there may not be a real easy way to put safeguards in place for every possible fungibility attack if certain kinds of fungibility attacks have not started yet and they remain mere speculation rather than actual and current practices.
So some of these FUD spreading nutjobs, like jbreher, who seem to want to continue to present the speculation of the fungibility issues as if it were a current attack on bitcoin, rather than speculation of the future seems premature, misleading and an attempt to spread FUD about speculation rather than actual and current things going on in bitcoin.