I didn't watch MoS, so I can't comment on any de-powering that went on there.
My initial response was to your assertion that it was close fight after the K-Arrow. My assertion was that it wasn't a close fight after the Kryptonite.
I see what you mean, but I would still content that even though it appears Superman would have lost if not for the Batman gambit, its still a pretty close fight in relative terms.
As to Man of Steel, in that movie it was shown that Superman's powers come not only from exposure to yellow sunlight, but overall from Earth's (lower) gravity, its abundant sunshine, and its atmosphere. In an environment that replicates that on Krypton such as within the Kryptonian ship that Zod has, Superman's powers quickly disappear entirely. And when Zod tries to terraform Earth into Krypton at the climax of the movie, Superman was shown to be negatively affected by among other things the stuff the world engine was spewing into the atmosphere.
An interesting question is what that stuff was. Perhaps in the Man of Steel universe, the stuff that the world engine puts into Earth's atmosphere to replicate the atmosphere of Krypton is, in fact, that universe's Kryptonite. Maybe there's something beneficial about it for Kryptonians, but it interacts badly with them when Kryptonians live for extended periods of time on a planet like Earth.
Incidentally, the introduction of Doomsday opens the door to a theory of mine I've tossed around regarding an otherwise weird plot hole in Man of Steel. If exposure to Earth gives Kryptonians superpowers, why would Zod want to terraform Earth into Krypton? Wouldn't it be better if they kept Earth earth-like, and even if there was a period of suffering the net result would have obviously been great for Kryptonians. It seems like Zod is pursuing a strange goal; the goal of depowering his own people.
It might actually make sense when seen from the perspective of Zod and Kryptonian culture as portrayed in MoS. As Jor-El seems to imply when he describes Kryptonian history to Clark and Zod also seems to imply through his actions, Kryptonians were very conservative in their beliefs about everyone having a specific pre-ordained place in society that was dictated by genetic programming. In their own way, they believed in a genetic purity of their species. If so, what if the mechanism that grants Kryptonians their powers on Earth is not simple adaptation, but something that happens at the genetic level. Their actual genes adapt to the new conditions and cause them to in effect mutate into something that can thrive in their new environment. Superpowers would be a kind of genetic mutation. If so, then that might be the reason Zod isn't swayed by superpowers. In his opinion, Kryptonians should live the way they were meant to live, which is in a Kryptonian environment and a genetic adaptation to that environment only. He might also feel that a world full of superpowered Kryptonians would be disruptive to order, but the notion that he would also have a racist reaction to anything that "changes" Kryptonians in any way actually makes sense in the story.
There's no basis for the hypothesis that superpowers come from a genetic adaptation anywhere in MoS, but Doomsday might provide that. In the original canon, Doomsday was the product of strange, Lamarckian experiments with Kryptonians, suggesting that Kryptonian biology isn't like most: their genetic structure can adapt to harsh conditions in unconventional ways. If Doomsday establishes a similar fact in BvS, that Kryptonian biology is adaptive at the genetic level and can produce a being like Doomsday through experimentation, then that would connect to the theory that Zod didn't see superpowers overall as a good thing, and would be motivated to terraform Earth into a planet that wouldn't grant Kryptonians superpowers (or at least as much superpowers - the Sun would still be stronger) because it was far more important to Zod to return Kryptonians to their predestined state.
