If regulated businesses break fungibility, then people just need to start using decentralized apps / services to go with their decentralized currency, then it won't be a problem because individual human beings aren't going to query the history of coins before accepting them - only big business.
Name some decentralized apps and services that allow people to lock in their Bitcoin value or exposure to volatility.
Tether USD over bitcoin protocol, Bitshares pegged assets if you don't mind moving off bitcoin. Anyway, even if what you want now doesn't exist, doesn't mean that it won't exist in the future.
Tether is issued centrally and extremely exposed to centralized authorities. Bitshares is not Bitcoin (though I don't know much about it in general).
They may be forced not to accept certain marked coins e.g. from a state published list.
Penalty could be criminal prosecution.
all coins are marked
Yeah. But certain coins may be marked as coming from illegitimate sources.