Not only this, Tanjirou still needs to get to Pillar level (Inosuke and Zenitsu too I guess) and there's Kanao and Genya too, I'm convinced that either there's going to be a big purge of characters coming soon or Muzan will somehow introduce a new batch of enemies to the plot. Another posibility would be that aside of pillars other characters might dissapear into irrelevance, but I don't think people would be too happy with that (and I find it unlikely to happen as well).
I know but the arcs have been structured as UM vs Pillar so far, even here we got two upper moons with only two pillars to face them (unless others join) Kanao and genya could be incorporated into shinobu and sanemi storyline (or take their pillars spots when their teachers die rip). really if rock and snake didn't exist I'd say we reached the climax. Anywho the author has succeeded in implementing "no body is safe" how relaxing
It will feel cheap to get new, even more powerful enemies all of sudden, especially since muzan killed the lower moons, well we'll see if this happen it might turn out good idk. I have high hopes for douma his design is cool and have been around since daki/gyuutarou were human, and well #1 is #1 and a lot of mysteries, akaza got flushed before so maybe the last 3 will be catastrophic, heck hantengu isn't dead yet and he's above gyokko.
As for the main four, I still feel they're far from a pillar, give them a couple more fights. but since the most deadly ability of UMs so far is poison and tanjirou carry a cute incinerator on his back he'll be op, which I also wonder if she'll have more power ups or if tamayo will give some information by the end of this arc with all the blood she's got.
I agree it's pretty early to jump to conclusions though. If they're gone for a couple more arcs then we'll talk
To ease your mind, they're lurking on the inside cover and vol.12 introduction page, they haven't been forgotten xD add to that zenitsu is ranked #2 in the popularity poll no need to worry about him. More importantly if he sees Kanroji A] explode B] become the strongest pillar,kill muzan, marry her. story end. They can't be in the same place.
Btw just sew vol.11 omakes, so there was a story for Kanroji uniform xD Imagine kanao burning the thing with blank smile