Welcome to the Downs, Smaug!
The threat to the Free Peoples of Middle-earth should Sauron have regained the One Ring was very real. Sauron's defeat in the Last Alliance was a difficult task, accomplished only after 12 years (counting from the taking of Minas Ithil) of battle and at great loss to his opponents. Arguably, the powers of the Elves and Men had diminished in the 3000 years between Sauron's defeat and the finding of the Ring. A military defeat would have been even more difficult, if not impossible, with a re-Ringed Sauron, leaving all of Middle-earth under the Shadow again. So, indeed, only the Elves would have been guaranteed a place of refuge, assuming the Havens remained open after a Sauronian victory. Pretty dismal for the poor hobbits, non-Sauron serving humans, and dwarves.
As to Eru directly intervening against Sauron, that really doesn't seem to be his style. Interventions appearing in the LOTR seem to be more along the lines of 'chance' happenings that give the intervened-upon individuals the choice of acting in accordance with Eru's plans or not. If Eru just came in and sent Sauron packing to the Void, that would deny humans, as the ultimate inheritors of Middle-earth, the chance to reject Sauron on their own. Additionally, it seems inconsistent for a being who admonished Morgoth that his actions would, in the end, only serve Eru's purpose, to step in and undo the opposing actions. That's almost as if Eru is saying "oops" and admitting that Sauron can interfere with his plan by misbehaving. And Eru never did directly oppose Morgoth's actions, even though Morgoth was a more powerful threat to Arda than his lieutenant. I'm not saying that Eru couldn't intervene - it's just that such an intervention doesn't fit with my understanding of Eru's typical behavior in such situations.
I hope that makes some sense. I'm not sure that I make sense to myself when I try to enter mind-bending philosophical discussions.