Void Elves are the answer to the request for playable thalassian elves on the Alliance. They may not have been the answer you wanted, but they are the one I told you for many years you might end up getting, a variant.
Of course my theory was that you'd get sentient wretched but the gist of my point was proven right, that Blizzard would not grant the Alliance a duplicate of a core Horde race.
The Void Elves are a compromise. I am not keen a model I felt was Horde unique ended up being shared as it has been, and I feel the same way about the Nightborne which should have been Alliance unique. However, that is done, and it is a mercy that both Void Elves and Nightborne are as far thematically from their parent races as it possible to be. They can at least be their own thing.
In regards to the overall thrust of this topic, I believe that the Silver Covenant should be phased out. If we are lucky, the constant High Elf spam on the forums here and on the official side might have provoked Blizzard to write that into a future story point, but I doubt they are going to be moved by forum spam into doing something like that. Although poor High Elf Archmage Ilira WAS converted into Blood Elf Archmage Ilira during the beta and it wouldn't surprise me if the forum noise made Blizzard realise they were being too liberal with their use of High Elves, and so scaled back.
As for your theory, the Void Elves are heavily implied (without it being directly stated) to be able to convert other thalassian elves into Void Elves. Sure, it's only implied, but the primary objection to the implication is not rooted in any real lore reasons, but the sense that if the Void Elves are able to increase their numbers (which they likely are) then the population issue objection to High Elves has not been invalidated by the addition of Void Elves. Worse, given that the few remaining Alliance High Elves are the likeliest source of new Void Elves, admitting Void Elves can increase their numbers means accepting the already tiny pool of High Elves remaining is decreasing even further.
The only real objection to your theory I have is your belief Blizzard will convert a large chunk of Blood Elves into Void Elves and give Silvermoon to the Alliance and destroy the Sunwell. That is not going to happen.
The Blood Elf-Void Elf storyline is going to be THE storyline of thalassians for the future. The Void Elves are infused with void energies. The Blood Elves draw on the light via the Sunwell (note this does not make Blood Elves something mad like Sun Elves. That would be ridiculous, laughable headcanon. Blood Elves remain High Elves). This mirrors the upcoming struggle between the Void and the Light that is going to dominate the game for the next couple of expansions. For this dichotomy to work, the Blood Elves need to keep drawing on the Sunwell. And to keep drawing on the Sunwell, they need to keep it AND Silvermoon city.
Not to mention the gameplay necessity of the Horde maintaining a city in the eastern kingdoms. Same reason the Horde isn't conquering the Exodar.
Far likelier is the Void Elves absorb most of the High Elves in the Alliance and that makes them large enough to be relevant but not overwhelmingly so.