Well if you wanna be technical, if even a third of the dead from the sacking of Quel’thalas joined the Forsaken they’d still have 3 times more Thalassians than all living Thalassian factions put together not even counting losses in subsequent major conflicts so the number of Elven Forsaken is likely much more significant than what has been generally showcased, which in my honest opinion is a shame given how drastically diverse the undead of Warcraft have been.
As for the “Why should they do it?” debate, for one the Forsaken fanbase got screwed over the past 3 xpacs so hard it’s not even funny to see Night Elf fans complain. So throwing them a bone could go a long way. Also if we believe some of the rumors and/or clammering for more Azeroth based stories the fallout of the power vacuum in Tirisfall would be ripe for storytelling between the Scarlet Crusade and their secret heir, Turalyon and of course the forsaken, especially if we believe the timeskip rumors.