You're right - the Wowprogress data is more current. That does show a disturbing trend for the Alliance high-end raiding scene.I don't know how did you check, but I can tell you about my region (EU English servers):
PG - moved to twisting nether horde
Bleached Bones - is horde
Group Therapy - moved to kazzak horde
Alterac Deviants - moved to twisting nether horde
Inner Sanctum - still alliance
Polar - disbanded
Honolulu - still alliance
Arctic Avengers - moved to kazzak horde
Unnecessary Flame - disbanded
Whatever - disbanded
Member Berries - disbanded
The Inner Circle - still alliance
Eversio - still alliance
Halo - moved to tarren mill horde
Strange Behaviour - disbanded
Avalerion - still alliance
Loot FTW - moved to draenor horde
Consilium - disbanded
Eternal Shadows - still alliance
No Hard Feelings - still alliance
Phoenix - still alliance
So out of 21 guilds in Hall of Fame in my region from Uldir, 7 went horde, 6 disbanded and 8 are still alliance. That's about 1/3rd of the guilds surviving 1 expansion. What kind of goddamn lie is "I could only see one guild that transferred to Horde, and only a small handful who've disbanded" what kind of data are you even looking at? I went to wowprogress page of each of these guilds and that's the info they come up with in the present day.