Or, just having an underworld as a background element facilitates the story point of "how do the badguys get their stuff?" Just because you know there's a mob doesn't mean the story has to include taking the mob down.
Yet, this will slow down somehow because we saw some of Roger's journey. It doesn't add up
You are completely missing the point. Oda doing Roger's flashback fast doesn't mean "he's going to skip other story elements because of Roger." But how he's handled it, the number of endgame details he's putting into play sooner than expected, (rather than doing this same flashback right before the crew gets to Raftel), the specific hooks in play that indicate we'll get to Luffy finding out these things sooner than later, combined with his earlier announcement, and other recent chapters like finally revealing all the Emporer bounties so we now know what the final benchmark is that Luffy has to pass, the existence of Im, even Luffy becoming an Emperor BEFORE fighting Kaidou, Shanks basically promising he'll meet up with Luffy after this arc is over, all show Oda IS eyeing that endgame now and starting to push harder towards it.
I expect we've probably still got a year or so of battles left to do in Wano, and then after that we have Elbaf (and the conclusion of all the lingering Big Mom threads) , Raftel, the final war and Blackbeard, epilogue, and whatever else Oda deems mega important, like the revolutionaries, Shanks, Vegapunk, Im… and presumably at least one surprise location we haven't heard about because knowing the path of the entire new world a decade in advance in "boring". . That's still a LOT to do and not a lot of time to do it in if Oda really seriously thinks he can manage it in five years to the point he's stated it twice. In the wake of still needing to address all those big things, some smaller stuff IS going to go by the wayside.
Just look at how the Reverie was handled. We got the setup, none of the actual meat of it, and are just hearing about the aftermath second hand. That was something that could have been six months of material on its own probably, that was completely skipped over in order to keep the story moving and the focus on Luffy.
Even if its impossible for Oda to hit his five year goal, and it instead goes for 8 and the 30th anniversary instead, thats what he's gunning for now and that's the lens we have to also view the series through from here on out which absolutely changes expectations and theories. "How important is something to the entire story reaching a satisfying conclusion" has more weight than it used to and some old expectations and hopes need to be brushed aside.