Now there's a man who's not happy with his editor!
Actually, I've a feeling that some variant of that has been said by more than a few writers about more than a few editors ever since....
And once again, to drag the thread back on topic.... I think the main problem with anyone else doing a "sequel" or even a parallel story, is capturing the feel of Tolkien's world and work. Just as with most writers who have a very distinctive feel, in order to do anything approaching justice to a tale set in Tolkien's world, you'd have to capture something of Tolkien's essence -- his worldview, the flavour of his style, something of that nature -- and I just don't think anyone is likely to either be able or be interested in doing so (or both).
With Lovecraft, even before his death, what became known as "The Cthulhu Mythos" was becoming the joint product of many writers, which gave some leeway in atmosphere and approach. (Writing a
Lovecraftian story is quite another, and much more difficult, thing.) Even so, the
successful attempts are comparatively few.
As for the later Foundation novels by writers other than Asimov... there we did have not only the estate's permission, but also (iirc)
Isaac's own approval of
their own work, which may give some (very slender) rationale to such a business. With Tolkien, this isn't likely to be the case, considering his dim view of others mucking about with his work. (Again and again I am reminded of Lewis' famous comment about influence: "No one ever influenced Tolkien -- you might as well attempt to influence a bandersnatch!" I'd say the response to others attempting sequels or spinoffs to his work would get much the same reception....)