Complaints which are unjustified? I'd say they're pretty justified regardless of how low they might be on the developers priority list, although I find your notion of "Not in launch = not a priority" to be utterly absurd. For a single player game, sure. For a MMO which is going to receive updates for years to come? No. Not even close. Content that wasn't in at launch wasn't a priority for launch. Wolves Den was a priority for 2.1, Frontlines was a priority for 2.3. Slaughter was a priority for whatever update that was, and whatever the new mode is called is a priority for 3.0. The fact that PvP, like Housing, has its own team to work on it alongside other content means it is a continuing priority.
Frontlines isn't the only PvP in this game, you know... I really do dread to think how the common player is going to react to 3.0 if the handful of PvP skills confuse them, none of the PvP ones are even used in combos and their effects are very clear (this removes debuffs, this heals you, this is defensive) but that is an entirely different debate... Still, if the new PvP skills are like driving a tank to some players... That is a new low for this games playerbase...
Or they could just limit where we can Duel, like has been suggested numerous times, along side just throwing it in the Wolves Den instance; Making a 1v1 mode for that wouldn't be a massive undertaking in the slightest. Can already use the PvP skills at Wolves Den Pier anyway, why not just add a bit more to that map (again, they're making an entire new map for Frontlines so it's not like they couldn't have) and make that the designated Duel area? A Duel area doesn't even need to be significantly complex, it would be a very easy area to design compared to what we've seen of the new Frontlines map (which looks great even if I have my doubts about the mode itself).
The issue isn't about popularity. It's about access. Go back a few posts and read my ridiculous rant, the issue with PvP is that people cannot get into it due to the queues, which in turn makes the queues worse, which in turn is not helped by splitting players across multiple different queues (Wolves Den, Secure, Slaughter, etc.). By not addressing this simple issue they are effectively damning every piece of PvP content they release. Wolves Den was popular, then that little negative feedback loop kicked in and it became very time consuming just to get in. The exact same thing happened to Secure, and Slaughter. There are other issues, sure, but this is the main one.