One piece definitely lost a bit of it's appeal since TS.
But it's not that story itself has issues but because of some poor ideas and their execution.
Separating SHs in first major arc since TS until wano is one of them especially when we consider that Shs were separated since sabody.
Then offpaneling important characters and stuffs for something less important or fake hypes which aren't executed well or dragging of sub plots (wedding cake) are few of other reasons.
Also, oda has maxed out world building of one piece to such extent that no matter what he does he won't be able to do justice with all characters now.
But most importantly I think oda isn't realzing that a mangaka must adapt to the changing times and circumstances to keep the story interesting (definitely not an easy task but you expect something like this from Oda who is no doubt one of the best out there).
It's disappointing run in recent few years but I won't call it mediocre.
One piece lasted at top for years and it's not something medicore manga can achieve. And, one must realize that's it's really difficult for anyone to maintain peak performance for 2 decades and still running.
And, story wise (not if you are invested in PL more) one piece still has those elements that can hook you up.
So, I agree that oda can easily push one piece at back of its peak after wano because story post wano is befitting for what oda is quite good at.