If the current generation of Jump readers wanted a dark series, wouldnt there be one in its current lineup? Wouldnt the other series released in the past year and before, and even before that still be running? Other magazines out there have darkish series so it is reasonable to believe that people are able and comfortable with getting their dark fix elsewhere.
I always found it weird how we haven't seen any dark series for years up until this point, like the editors were trying to steer away from the genre as much as they can in favour of romcoms, sports and gag series.
I can even pinpoint the moment when darker series fell out of favour to the romcom. That was in late 2011 when the last newly serialized mystery/suspense series Enigma got replaced by Nisekoi the following week, the first highly successful romcom since Hatsukoi Limited, which ended in 2008. It's why I've gotten jealous of romcom fans with subsequent romcoms getting released one after the next, and especially because this particular series seems to have its own, dare I say, Nisehards, on a grander scale than any other popular series I've seen.
So for Shiibashi to return with the first dark series since Enigma is going to be quite a breather, and I hope its genre follows the growth of popularity as the romcom.
Speaking of Enigma, if Kenji Sakaki returns with a dark series, which I'm pretty sure she will, I hope Illegal Rare won't suck away its competition, if the two series were to serialize simultaneously. I'm hoping the two series would compete with each other in the same vein as between Kuroko and Haikyu rather than between Nisekoi and Koisome/Pajama/Edison(/iShojo).
Am vastly looking forward to early impressions of Illegal Rare vs ToC (unless World Trigger, Haikyuu!! fell to the bottom

)
You do know that for the first 7-8 weeks or so, the series is placed at complete random, right?
The occasional rare success doesn't change the fact that successful dark series in Jump are usually an anomaly.
Successful Jump series usually always have a core theme of friendship, teamwork, and/or adventure.
Dark series tend to sacrifice one or more of those elements for the sake of being edgy.
You're referencing the three principles for WSJ manga to be popular right? Friendship, effort and victory?
Having read Enigma, I can safely say each of the three principles were extremely prominent throughout the series, at least within its first arc. Too bad the second arc couldn't live up to it, and I agree that the principles got thrown away altogether in that section of the story.
Having read Nurarihyon no Mago, I can say the same thing, with the concept of the Hyakki Yako, the training, and the teamwork.
So just because a series is dark doesn't mean it automatically sacrifices these elements.