Tanjiro is way stronger than the pillars right now. Remember that the pillars were having a hard time even when all of them were fighting him. So that means each individual pillar was getting less attention than if he were alone facing aginst muzan. Therefore, tanjiro, who is getting muzan's full attention, is stronger than the pillars.
I don't really have a problem with the aging drug plot device. But its strange that all the pillars got done for so dispatched in such a short amount of time, especially when they were actually managing to hold back muzan. It feels forced, and done solely so that tanjiro can get center stage.
I really enjoyed binge reading this manga. The only thing I found slightly uncomfortable was the pacing, which was all over the place. I don't know how the manga industry works, or if the author was forced to end this in 200 chapters or this much pages. This is the kind of manga where you can take your time and flesh out all the characters. Then you don't need to rely on flashbacks (especially near death ones) so much in order to develop characters, and more characters can be fleshed out.
Also, I think the side kick characters should have gotten more glory.