Beyond the average Kaldorei, Sylvanas doesn’t think upon how the Night Elves, specifically their leadership, would react. The only thoughts we have on Sylvanas for the reaction of an Alliance leader is Anduin attacking out of desperation, which she later admits actually unifies the Alliance:
- Anduin Wrynn would have lashed out in a final, desperate war, looking for a miracle, because only a miracle would save them.
…
“They will come for us now. All of them!” he said.
“I know.” She was calm, as though nothing were wrong. “They will attack the Undercity in retaliation. You will need to plan our defenses. Begin evacuating my people.”
Which goes back to Spuddyc’s original point that Sylvanas admits this is exactly what would happen if the Horde tried to win the war dishonorably.
The Night Elves would be invigorated into taking back Teldrassil, not taking Lordaeron or Gilneas. This does not factor into Sylvanas’ opinions of Genn.