Then you can nerf scourge speed to 3.75. It’s that easy.
If they have 3.75 movespeed they can kite them fine. As could voids and mutas.
Well they have the capacity to kill the scourge before they deal any damage, thereby protecting the air units. Thors, mines, storm, pb, and fungal would all wreck scourge before they deal damage if you cannot get a sufficient count of liberator, corruptor, or phoenix. Do you think scourge would keep its 100+ bw dmg or something? You can easily bring it in line to baneling levels, 20 (+15 vs light).
They really can. 3.75 movespeed and 20 (+15 vs light) damage, 25/50 cost, 0.5 supply, morph from zergling with spire tech. There ya go, it’s that easy.