Jump+ isn't a dump for failed WSJ manga, it's a separate publication with its own hits. It doesn't need Agravity Boys.
I think there's only ever been two series that transferred to Jump+, other than for an epilogue? One of them was by choice (Tomatoypoo), though I don't know why iShoujo switched over.
iShoujo was canceled on Shonen Jump, had a special chapter on Jump Live and a full sequel on Jump Plus because the manga was popular online. Soul Catcher(S) also moved from Shonen Jump to JumpNEXT and to Jump Plus due to popularity and good sales.
I think your choice of words were quite poor, WSJ isn't the ultimate place in the manga industry where if you can't succeed there means you're trash with not place, series can grow in another platform, Agravity Boys sold 12k in a month, this is more than a really good part of Jump Plus, so why they wouldn't need it? in fact i think it's the perfect platform for the series, it could grow there without having to worry every week about rankings.
Also, Jump Plus is like Magazine Pocket or Sunday Web, these platforms have they own series but they also are used sometimes to transfer series from the main magazines when they aren't really working there, isn't any demerit as you're trying to make it like.
