I completely understand why you would say this is something that needs to be changed, I really do, but there's another way you could look at this.
If you did reduce the damage by half, that would make parrying while oos completely worthless. Let's say you're oos and you parry a heavy, you still get nothing as usual. I believe nobody gets anything by parrying heavies while oos. Let's say you parry a light, every character guarantees a light on that parry.
If you reduce the damage by half, parrying that light is no longer worth anything except for stopping your opponent's chain. Most light's do somewhere around the range of 16 damage normally, which isn't bad. Reducing it to 8 makes it not worth the risk of getting parried oos and getting smacked for somewhere between 40-90 damage.
Let's go with the heavy situation. Dodging while oos is inherently dangerous, even for assassins who normally have the additional safety of a dodge attack if they react or predict well. However, dodging an attack with a long recovery (such as a heavy finisher or some unblockable melee attacks) is an excellent way to get a free oos gb. Normally, most characters would at most get a light out of this. However, if you stand near a wall while oos, you throw in the possibility of getting a wallsplat from a throw into a free heavy. This can do anywhere between 25-45 damage depending on who you play.
Reducing the damage by half makes this risk/reward significantly skewed toward risk, which shouldn't be the case. Yes you should be vulnerable while oos, but you shouldn't be helpless. Regularly you've got about a 50/50 risk/reward balance. While oos, you're at about a 70/30 risk/reward. If you reduce the damage by half, that would probably go closer to 85/15. That's a bit too much.
As much as I want to hate this idea, I love it. The whole idea of taking full damage if you can't dodge an Oos attack is preposterous. You're out of ****ing stamina! You should not be doing full damage as slow as your moving.
On a side note, guard breaks should be 40-50% stamina cost. That would keep people from spamming it.