Problem: Chaos and Black Kat armors work for all situations
There really is no reason to not take the easy max damage and ctr, with black kat offering movement speed on top of that. in a game where you can complete endgame content with minimal effort, making it easier is pointless, so players choose to make it faster instead.
Solution: Make other sets even better, but only for certain situations
Let's take the Mad Bomber suit as an example. in a vacuum, a fantastic choice for arena levels, as you deal with large groups of enemies there and bombs can wreck havoc on them. however, why limit yourself to bombs when your sword is also buffed to the max? I suggest the Mad Bomber suit should give damage and ctr for bombs above that of chaos/bk. this way, your bombs can deal with enemies just as fast, if not faster than a standard chaos+combuster combo.
same logic can be applied for any other weapon type, sacrificing the effectiveness of other weapons should let you improve your weapon of choice beyond what is normally possible. additionally, more armors could be granted such abilities, for example Angelic Armor and its derivatives, on their own could provide buffs, as well as increase the maximum possible bonus, allowing players to specialize with trinkets and UVs
Specialized armor should provide bonuses beyond regular Maximum!
so, its making drawbacks to have better stats on other elements of a set? They could even use this to create more armor sets whit wild stats, preferably based on fan content given away by the community for the desing.
Would love more especialiced sets based on other monsters or themes, taking these drawbacks has something a player would want to deal whit in exange of better perfomance, since being untouchable, during a fight, somewhat jeopardices defense and debuffs in general. A choice for Ace players whit more varying playing styles.
Btw would love to see an Oiler set armor whit shock drawback and hibrid resistance to pierce and elemental as well high resistance to fire (ofc).
Besides the fashion, drawbacks and bonuses are really funny things to consider when dealing whit higher challenges.
I think a more reasonable approach would be to just give more Armors negative status stats in order to raise the bonuses they give.
The baseline for 5* Armor is basically a 3-point system:
1 status costs 1, reducing 1 status adds 1.
1 stat at low costs 1, reducing a stat by low adds 1.
There are some special things to this, i.e. family damage costs 1 but gives medium.
A very basic example is the Skolver set:
Cold Defense and Two Sword Damage Low are 3 points.
For an example including a negative status in order to gain 1 point, I'll go with Sacred Firefly Shade:
Electric Vulnerability, Fire Defense, 3 stat gains, 3 points.
Taking this point system into account, I personally would really just suggest adding status vulnerabilities to more Armors.
Let's give the Skolver set Poison Vulnerability in order to get another point, then use that point to raise the Sword Damage Bonus to High. With this, you get Sword Damage Max if you wear the whole set.
If you want to take this further, let's add an Electric Vulnerability to also give it Sword CTR Low.
If you go over the whole roster of Basic Hall of Hero Armors this way, you get a much better result than breaking the game's core system just to make BK look "worse."