On a downed target, skills with KD+Downsmash get instantly broken by the downsmash AND makes the enemy CC immune!!!! I main Succession Sage, both my KD's have downsmashes, what am I even supposed to use after catching an enemy with CC?

The problem:

How long a Knockdown is (using 2nd rabam):

idk what your problem is tbh, get a brain and learn how ccs work

Class with no knockdowns that don't have downsmash in them.

Doing follow-ups for my teammates in AoS basically griefs my team by ruining their combos.

And in general, knockdown should always take priority and not break.

So basically you are just unhappy with sages's moves?

the attacks are designed this way, with no oversight on how this is supposed to work. 

Not too sure what exactly you are complaining about OP.

The skills work exactly as they should and what you show in your clip is not caused by downsmash at all.
Downsmash on KD skill is an issue for a completely different reason, but I will go into that in a bit.

What you did there is working as designed.
You use Ator's Mark - bound followed by Ator's Fist - KD.
Naturally the CC counter is capped... you just used 2 different HARD CCs.

Also the NPCs in the arena do not stand up as fast as players do and last I checked they can't resist smashes as well.

With that said the only issue with a smash on a KD skill is that if the KD is resisted the enemy may still get smashed, so if you aren't paying close attention you will be continuing your combo, but they will be back up in 1s and not 3 and counterattack/run.