[Lonestar]

I don't believe Black Knight, as a standalone class is very strong as you state it is. It has strong tools, but the biggest power in spike is when BK is combined with classes that doesn't require any of the skills and is simply used for the passives alone, which it not particularly healthy when the class has so much more to offer. Combos that are very famous out there can go from BK/Priest, Hexer/BK, LB/BK, Summoner/BK and much more. You become tanky simply by having +6 DEF, a parry that works on your front/flanks against three types of weapons and free crit evade.

Black Knight as it is, is around the same problem Boxer has, which it is only used thanks to a gimmick, which in the case of Boxer is their Geist, but also differently, Boxer offers so much more compared to Black Knights in terms of different skills and offensive capabilities, to the point that they match their theme. Black Knights are not frontline warriors or bulky protectors, they're in fact, both, to the point that they have skills meant to support your team through sheer guardians like Castling or Soldier's Protect. Lorewise they're both used as Chaturanga's army to defend their home and mercenaries-for-hire that can range from jobs such as bodyguarding due to their increased defenses. They're the game's best physical tank so I don't see why they could not be that role.

As it is, I don't believe  they're a case of 'suffering from success'. They have strong tools and underused tools, and not all the skills and passives matches what the Black Knights usually built for stat-wise. Some of the powerful skills have cooldowns and others can be kited around, alongside one that requires a very peculiar positioning (looking at you, Bright Bishop). The changes are very much welcomed and BKs, and it could change how people approach it, instead of being a forever sub-class option.