We have evidence in this thread of users not wanting to accept Segwit transactions. That's users not miners, that's a lack of fungibility, and that's an issue.
I didn't know that. If you gave me a legacy BTC address (if we had some sort of deal or transaction), I'd send you some BTC that came from a native segwit address. You'd still get your BTC, and you'll likely get it confirmed in a block faster too. Why would you not accept that? Would you return it?
Its a crazy idea, but it is true. There are some Bitcoiners out there that will not accept a Bitcoin TX if there is any segwit TXs in its lineage. Take this with a grain of salt, as my understanding of this attack is very basic and I have no clue how viable it is. The idea behind not accepting Segwit TXs is because there could be a potential 51% collusion attack in the future, where the miners would roll back the Segwit fork and accept all the past Segwit txs as anyone can spend. They would then continue mining the original protocol satoshi designed. So theoretically, there would be two chains. The legacy BTC chain, and the rolled back Segwit chain with all the stolen coins. Any "Legacy" BTC that has no Segwit TXs in its lineage have no risk of being stolen in this attack.
Again, like I said, I have a very rudimentary understanding of this attack. I don't really know the specifics or its viability. This is something that Anonymint has been talking about on this forum recently under his alt account "anunymint".
If the practice of not accepting coins that have any segwit anywhere in their history is actually true, then I would concede that lessens bitcoin's fungibility; however, first of all this seems to be a fabricated set of facts... nonetheless if folks are merely playing such nonacceptance games for political reasons, then this group surely would have to garner support for such issue before such lessening of fungibility becomes something worthy of attention.
Still seems like a FUD spreading phenomenon to me (and the topic of fungibility seems to be a large exaggeration rather than based on any kind of meaningful issue), rather than anything that is likely to become a real and meaningful problem.
Edited: Added response to bitserve
About fungibility.... if one (jbreher?) wants to take the term to the extreme meaning (IDENTICAL), then that doesn't really exists. There's no two identical anything. Not in cash -bill denomination, serial numbers, usage wrinkles, etc) not in gold bars -different weight, composition, usage marks, etc- neither in crypto, yeah.
Of course this fungibility is not an issue, until either a government or individuals begin to engage in a practice of changing value of coins based on demarcation, which is currently not an issue, but instead a speculation of an issue that could come up perhaps maybe blah blah blah. And, also assuming that there is no way to remove history, then there is a way that coins could begin to look distinguishable from their history... but so far remains a BIG SO FUCKING WHAT?
But it is common sense to use the term as SIMILAR/REPLACEABLE and in that respect there is absolutely no lack of fungibility due to Segwit as jbreher is saying. That's complete bullshit.
You seem to have identified the problem, and that is jbreher trying to distort issues.