I wouldn’t say the Forsaken are ‘gone’, but there’s some troubling times ahead for them.
No Val’kyr:
No resurrections. No raising the dead en-mass. Only powerful Necromancers can do it now, and that’s on a case-by-case basis … and after the War of Thorns, very few people are going to want to willingly be raised into Undeath, leaving only the sick, the desperate, the mad and those terrified of what waits beyond the Veil.
None of these are going to have sturdy bodies. None of these are going to come through with their minds unscarred and stable. The era of Forsaken soldiers covering the battlefield is gone, and the Desolate Council is ruling over a people slowly and quietly decaying away … unless Califlower has some tricks up her sleeve we’ve yet to see.
No new spare parts:
Forsaken could, with a bit of medical knowledge, some alchemy and some necrotic energy, attach the limbs of Men, Elves and Dwarves to their frames to replace those damaged by conflict, or to gradual wear and tear, since Forsaken don’t heal naturally like the living. Any wear, any damage, is permanent and never truly recovers, even with the fungal aid from the Plaguelands and their Cannibalise ability. At best they can slow it down to a trickle that might allow a Forsaken to endure for decades, if not centuries.
What we’ve learned in the Shadowlands might alter this, especially if the Forsaken can figure out how to replicate what was done to Nathanos without needing to sacrifice the Living, and become Undead entities that possess the same regeneration and restoration abilities of the Living … and possibly the ability to reproduce naturally too.
A low profile:
For better or worse, Sylvanas took their name, already considered dark and untrustworthy by the Light-worshipping masses of the Eastern Kingdoms, and ground it hard into the dirt, then washed it off in the contents of a chamber pot and did it again. Much like the Orcs struggle with the perception that they are mindless brutes and inveterate warmongers due to the lingering horrors of the Original Horde, the Dark Horde of Blackrock Spire, and the efforts of Garrosh and his ‘Twue Howde’ nonsense, the Forsaken are irrevocably tarnished in the eyes of Azeroth, and this is unlikely to change for generations to come, if at all.
Any large gathering of Forsaken is going to earn a fair share of alarm and hatred from those outside of the Horde, and it is going to be back-breaking work for the Forsaken to earn back even a smidgeon of the trust and respect they once held in the eyes of the other factions and nations of the world.
Thar be Dragons:
There’s a shiny new race to play and, much like how the Orcs lost a lot of population post-Mist of Pandaria as people got turned off from the Thud and Blunder archetype being thrown around, and bad-faith actors declaring themselves Kor’kron and abusing the hell out of the other Roleplayers, the Forsaken are saddled with a lot of the atrocities of the Battle for Azeroth, regardless of their actual involvement, as a people and as individuals, and that’s going to hamper a lot of folks playing them for some time to come.