Oshi no Ko is a great anime:

From the emotional start with Ai's death to the startlingly raw and severe description of modern society Oshi no Ko delivers a fresh and important take on celebrities.  The artwork is equally innovative and expressive, drawing you in against your will.  And on top of that you have the addictive opening theme that's now all the rage worldwide.  Oshi no Ko qualifies.

I would not say Oshi no Ko is a great anime on the basis of just the seven episodes (really 9 since the first episode was 1.5 hours long) that have already aired, but we all know this series is getting a sequel sooner or later -- it's one of the highest selling manga in the world right now thanks to the anime boosting its profile, and series like that always get a sequel.  There's also already 100+ chapters of the manga to work off of, so the anime has plenty of material to base its sequel on.  If you think of Oshi no Ko in the long run, with a season 2 or 3 or 4, it will definitely deserve its great anime ranking -- and my top anime rankings are prophetic, they take into account the future.

I am not as big a fan of Oshi no Ko as most people are, but I can still recognize quality when I see it, there is something about this show.  Moreover, Oshi no Ko doesn't have to be as good as Shinsekai Yori to get into my rankings, all it had to do is beat Seitokai no Ichizon.

Seitokai is a series I've always felt ambiguous towards -- yes it had a polygamous seeking main character, but no all of his polygamous overtures were rejected by the girls involved.  I don't need ambiguity on this issue anymore, I have Kanojo mo Kanojo and 100 Kanojo.  The modern world has left Seitokai behind, it's strictly inferior to what has followed after its pioneering daring.  Now we have anime where the girls accept polygamy too and treat the main character male right.  I've wanted to replace Seitokai in my rankings for a long time, but no long series was stepping up to actually surpass it.  Now that Oshi no Ko with its four seasons or whatever has arrived, we have a series that's both longer than Seitokai and more apt in its social commentary.

Going along with the change in anime rankings, I also edited '100 Waifus' to include Ruby from Oshi no Ko instead of Mafuyu from Seitokai.  Mafuyu was only there as the representative of Seitokai, she isn't actually that good a girl in her own right.  Now that I don't need a representative from the series Ruby can easily take her place.  It's a shame because Mafuyu was a unique name and Ruby wasn't, but overall it's still a better fictional character hall of fame with Ruby in it.

Who knows, if they don't announce a sequel season to Oshi no Ko quickly, it could also replace Card Captor Sakura in my top manga hall of fame.  That would be convenient, two birds with one stone.