This explanation almost works, but even if only universes with minor changes can keep existing, then we would still have a potential for infinite universes. They'd just not stray far from the main one.
But then, when someone is corrupted in a given universe, he would just disappear from that reality, since "he" already exists in the Nether. Unless a single demon can be in infinite universes at the same time. Which only makes things even more confusing.As for how transcendent demons in the Nether function it can go one of two ways. Either on attaining true demonic status a mortal from a given universe is translated over to the Nether and his or her memories merge with their already demonic self, or the mortal in question is simply annihilated utterly as their "spot" is already occupied by a given demon (e.g. the case of Othaar/Socrethar in this case). Another possibility is that this merge is also universally transcendent, and that once a given mortal in a given universe becomes a demon then *all* their copies in alternate universes simultaneously do as well regardless of their current status.
But, if the titans only existed in our reality, then how could alternate Draenor (which was partially ordered by a Titan) exist almost identical to our Draenor? Without Agrammar, there would be no Grond, no colossals, no gronn, no ogron, no ogres and no orcs. Draenor would be entirely consumed by the Evergrowth.