Yes we want Corruption to be more viable. It's the least desirable healing spec to play in RWZ. And yes, we don't think Madness is too strong. It's just fine. Top of the leaderboards in Reg WZ and viable in Ranked WZ.
There are 24 trees and if this change happened you would still have 23 that could not cleanse your DoT. How does that break your class design?
And since you brought up Lethality, why can Operatives cleanse their DoT? How is that not game breaking for that spec?