Fighting games are designed around risk and reward, and characters are given a vast pool of moves to choose from for different situations. If one option has such a good risk reward ratio and is applicable to so many situations that most of the character's moveset barely matters, it's objectively bad design.
If that was your point all along, my apologies for misunderstanding. DK and Bowser are definitely overcentralized around their grab kill confirms nowadays, no argument there. But I do not agree that those options are, in and of themselves, problematic from a game design standpoint. It's that they
have to rely on them because kill confirms are especially a necessity for superheavies given that landing raw kill moves in neutral is not generally feasible, and the rest of their kits isn't up to par. Get past Ding Dong / Whatever-it's-called-this-week-Bowser-uthrow-uair percentage, though, and killing is a lot tougher for them. I'm all for removing overpowered and overcentralizing kill confirms from characters' kits (good riddance to Bayo, ZSS and MK's ladders), but if it's justified for anyone, it's previous-low-tier superheavies.
At any rate, this might be deviating a bit from the topic. Rage is a double-edged sword, after all -- it makes the kill windows for those combos
earlier, but not wider. If rage Ding Dong stops connecting at 80% then that's a blow against DK's gameplan as well. I guess what I'm saying is that yeah, it's dumb, but it's acceptably so given the context of the characters that have them.
tl;dr if someone's gonna have stupid-ass low percent kill confirms, let it be the characters that are total ass without them, not those that are pretty good regardless. If, say, Roy got some jablock-into-charged-fsmash at 60% combo, I know I wouldn't complain, the boi needs it.