Instead, (bitcoin) work against them by increasing Bitcoin's fungibility
Thats because bitcoin has no choice. It is a non-articulated, mono-layer protocol, so, as you correctly point out, cherry picking which market services to support has become part of their "fungibility" solution.
Pretty disappointing for something which is parading itself as a "cash" medium who's whole basis of existence is to be user-agnostic isn't it.
People who say that Dash should be appealing to one market to the exclusion of another are confusing markets for "Money " with markets for "Products". The two are distinct and the fact is that Dark markets do not want their own money because it would be next to worthless if it didn't have counterparty acceptance in the mainstream to give it value through spendability.
Why do you think the $USD is the currency of choice for dark markets universally ?
For a product vendor, appealing exclusively to a given market is a choice. For a monetary medium it isn't because it won't have value in one market without seeing adoption in another.
Thats why full-fungibility, Bitcoin compatible, transparent blockchains will always reach far higher monetary premiums than their obscured counterparts - because they are far more adoptable and create a complimentary relationship between "light" and dark markets instead of an exclusive one.
At the same time, Dash supports the privacy priorities of both through an articulated protocol which means it can afford to be "market agnostic" where bitcoin and obscurity-based coins cannot.
