Now, as for Lovecraft's own thoughts on this:
***************SPOILER ALERT!*******************
This is from a letter he wrote to August Derleth on the subject:
Now as to the end of the thing -- of course I'm not satisfied myself, but I am very oddly unable to decide whether more or
less definiteness is needed. Remember Arthur Gordon Pym. In my tale the shoggoth provides a concrete & terrible climax -- & what I wished to add was merely a vague hint of further spiritrual horrors -- as Poe hinted with his white bird screaming
"Tekeli-li! Tekeli-li!" I wanted to leave the
actuality of the glimpse very unsettled, so that it might easily pass off as an hallucination. Possibly I ought to have left it
vaguer still -- & then again I had an idea that the thing ought to be developed at full length -- perhaps as a sequel to the present thing, or perhaps as an expansion of that thing to full book length ... What the thing was supposed to be, of course, was a region containing vestiges of some utterly primal cosmic force or process ruling or occupying the earth (among other planets) even before its solidification, & upheaved from the sea-botto when the great Antarctic land mass arose. Lack of
interest in the world beyond the inner mountains would account for its non-reconquest of the sphere. But then again, there may have been no such thing! Those Others may well have had their superstitions -- & of course Danforth was strangely read, nervously organised, & fresh from a terrific shock.... Anyhow, what I did set down was a sort of weak compromise betwist the two ways I vaguely & ineffectively thought it ought to be"
-- from a letter to August Derleth, May 16, 1931, quoted in
The Annotated Lovecraft, pp. 329-30, n. 230
Incidentally, this isn't intended to persuade the ending was effective -- that's up to each reader's own reaction -- but just to provide information for anyone interested.