Think on this.

Even with all the buff now, one of the best team is water immortal, which uses 2-3 healers and can out heal all the damage that the enemy can dish out.

If buffs were less, thus there is even less damage, what would happen?

Are you going to ask for nerf to healing next?

Like you said, most people stack buff. If, take your example, 10% is shaved off every buff, then we are at 180% instead of 210%, which is a 15% reduction (180/210=85.71%)

On the other hand, if the heal buff is decreased, from say 40% to 30%, that's only 130/140, 92.857%, so more of the "power" is kept, thus nerfing all buffs would mean favoring the heal team.

On the other hand, if we nerf the heal more, then while it might be back to balance for buff stacking team, heal team might get countered by teams that does not stack buff but instead focus more on CC. As it were, if buffs aren't as strong, more people will likely turn to CC.

Also, I'm fairly sure the heal one is an aura not a buff. Thou I guess auras can be nerfed like buffs.

Furthermore, part of the immortal build is GNW sakura whose self heal is part of the problem. I'm not entirely sure if that's affected by heal buff.

Granted the game isn't particularly balanced to start with. But if you change a very common mechanic, it can cause really complicated chain reactions.