The core of the withering/decisive split was the video game Dissidia Final Fantasy (original PSP version) and the phrase "Initiative isn't rolled, it's seized." Beyond that it was an attempt to create a system where "Miss miss miss miss miss miss miss hit and now someone is grievously injured" is fun during the first part when it's still just miss miss miss miss miss. Because during a fight scene in a story, or even a real sports fight, that first half where each side is trying to put the other down and failing? Isn't boring. It's most of the fight. And you can see the initiative (in the colliquial, not RPG sense) shifting, with one side gaining advantage and moving the other side into peril, and the other side trying to defend themselves and turn the situation around. An interesting fight is a series of reversals -- someone has the upper hand! No, now the other side has the upper hand! Etc., until one side finally manages to create a situation where they have enough of an advantage that they can end it, conclusively, or perhaps one might say decisively.
This is also a hell of a lot more "realistic" than a fight system where each side repeatedly lands hits and there's some arbitrary threshold beyond which one side can't take any more and dies (because when fighting with real lethal weapons, the first blow that lands solidly is often the, er, decisive one), like takes on D&D where every point of HP damage taken is e.g. a stab wound, and it's even more "realistic" than a system where every miss means the state of advantage during the conflict hasn't shifted at all one way or another.
I would say part of the problem is that withering is a bad word to have used and we should have thought up a better one, because I can use "initiative" colloquially to refer to the ebb and flow of advantage during a fight and you know what i mean, and I can use "decisive attack" to refer to the blow dealt to end the fight after one side has fully seized the initiative and is now in a position to end things and you know what I mean, but withering means nothing and sounds like bullshit game-y nonsense. Holden did spend a long time trying to come up with a better term than that and never could, though. Sometimes that's how creativity works.