I found Planes Above to be a rather nifty book, with the various ideas and plot hooks and threw out there.
I find the Outsider issue interesting, for it helps establish the Astral Sea as a living place with a steadily growing population. Also note that Lolth, Torag, and the Raven Queen, who don't have Astral realms, don't suffer from the Outsider issue.
Planes Above does a provide an actual, lasting solution to the problem of the Outsiders; it's just that the solution, which is cast down Asmodeus and re-shape Hell (the only divine realm any and all Outsiders can enter) into a sanctuary for the Outsiders, is so monumental a task as to be barely conceivable for even the greatest of heroes to attempt it.
Which means the challenge is out there, for those who wish to prove that they can go beyond the impossible.
And I like that.