The only thing that will save the Alliance population is a Thalassian elf option that is equal or better than BElves in generic attractiveness.
The min-maxers who rolled BElf for the OP racials won’t be swayed, but the casual folk—the people who originally switched sides back in BC—may switch back to their original faction.
Blizz tried to correct the BElf-induced population imbalance by hastily tossing in VElves, but VElves are obviously not enough. Either their implementation as an AR is too onerous, or they are not attractive enough to counter the BElf imbalance, or both.
There needs to be a knockout, exclusive Thalassian look for the Alliance, and guess what? It already exists in the Silver Covenant NPCs. Keep the blue eyes on the blue side. Red, gold, and green for Horde side. Keep their looks distinct, don’t homogenize (or straight up swap over) the faction motifs/themes.