Why? That's just as stupid if I said that varian should have stepped down after garrosh was beaten. Many forsaken swore revenge on the LK, doesn't mean the majority ever planned to die again.
- - - Updated - - -
1. By assume control, do you mean where she rallies and saves the horde on broken shore, or where she arrives to the meeting and later is having lorthemar and baine agreeing with her?
2. Suggesting the forsaken would ever join the alliance is a joke I don't need to explain
Anduin totes had the power to send or not send them away in the time period that was actually being discussed, i.e. that of Garrosh' leadership.
The Battle for the Undercity where Varian was informed that the Wrathgate was performed by rebels who ousted Sylvanas and her loyalists out of the city and used that as an opportunity to "reclaim" (despite the fact that Alliance has no claim on Lordaeron whatsoever) it for the Alliance, went to Tirisfal without the Horde's knowledge, assaulted it from the tunnel while the Horde was besieging it from the front still unaware of Varian's adventures and after killing the small fry while the Horde was busy fighting a demon lord trying to summon Sargeras declared war on the Horde and tried to kill its Warchief and the ruler of Undercity? That assistance /s
The Forsaken have the most certain future out of all the "races". In fact, if they ever get to be at peace, they will "outprocreate" every other race. They are manifested into "adults" as soon as they are raised, they do not need food, air or water and they do not die.
Unless somethign catastrophic happens to them and culls their numbers, their numbers will grow steadily and that spells doom for all of the living races of the Eastern Kingdoms. With every human and elf that dies, a Forsaken is created and I'm not even talking of all the graveyards from all the previous wars that can be used for such a purpose.
They did though. Undercity was taken back by the Horde because of both the Horde and Alliance forces disposing of the rebel Forsaken group led by Putress and Varimathras. Jaina convinced the Alliance not to take back Lordaeron in the name of the Alliance and therefore the Alliance forces did help take Undercity back for the Horde.
No matter which way you look at it, this is what happened.
It was already taken by force, hence the highlighted retake.
As pointed out above, the Alliance in the end decided not to retake what was taken from them in the first place because one of the major Alliance characters decided it shouldn't be done. They chose not to retake anything, but only help instead.
The reason the forsaken's lack of procreation became a problem was because Garrosh was sending them into a grueling meatgrinder against the worgen with zero regards for preserving them since he gave less of a shit about the forsaken than he did about trolls.
If wars stopped, they wouldn't have much of a problem with their numbers.
Sylvanas was obsessed with revenge, but there seems to be plenty of forsaken who are quite happy doing their own thing and / or have gone a bit bonkers. There's not much reason for the forsaken to just give up on existence, at most Sylvanas should have been replaced by someone with less single minded motives.
Like you said, most of them are Lordaeron's people - continuing (un)life in their homeland is as good a motive as ever.