Zergs defense is only as strong as the weakest link. Make the queen invincible and zergs will still die to 4 hellions. That’s a separate issue to how poorly designed the bc is. The bc is a nexus of a dozen unprecedented abilities. Give any one of these abilities to another unit and that unit would be strong and valuable for that ability (ex: the overseer’s sight range). There is absolutely no reason for terran to have all these abilities packed into 1 unit.
At least half of these abilities should be outright removed from the BC, or at a bare minimum moved to other starport units.
Give Ravens the ability to teleport. Give the banshee the ability to shoot while moving. Give the viking the ability to yamato.