Re: Do you accept that Voyager "killed" the Borg as a major Trek enemy
No, I don't think this can be blamed on Voyager. It is more a problem inherent to the Borg as originally conceived.
Of course, to a certain degree, any antagonist in any kind of heroic adventure-style storytelling runs the risk of being neutered due to the fact that he/she/it is bound to be defeated, so this problem is not unique to the Borg.
There are strategies to prevent the enemy from being entirely neutered, one of the central ones being to keep the main or real adversary in the background as long as possible, allowing your heros to defeat only servants or emissaries. If your heroes have difficulty with these, then the true enemy can begin to seem more intimidating over time, not less. Right away there is a bit a of a problem with the Borg because they are non-differentiated to begin with, so it is hard to create that layered effect.
Another problem is that the Borg start out so unstoppable and single-minded that there is nowhere to go but down unless they are allowed to assimilate the Federation, which of course cannot happen.
Also the totally undifferentiated, totally faceless concept is very hard to sustain, dramatically speaking. Arguably it only lasted for two episodes, since Locutus already provides a face for the Borg at the end of the BOBW (Part I).
All of these difficulties are in evidence right away in TNG and of course in FC, so I don't think Voyager can be identified as the culprit, the show just runs into the same problem that the TNG writers run into almost right away.
(For an example of effective "layering," I would cite the LotR, which is a classic heroic adventure partially because it does this so well. Sauron is always in the background, and the basic premise is that he can never be defeated by force alone. Meanwhile the Nazgul and especially their leader, the Witchking, are built up to be extremely intimidating in their own right. Then we find out that Sauron himself is but a servant or emissary of the Great Enemy of the Elder Days.)
The Dominion is an interesting example as well. I don't think they were introduced as effectively as the Borg by a wide margin, but the idea is much easier to sustain. Sisko closing the wormhole with the aid of the Prophets creates a sort of layered effect in that the rest of the war is only being waged against a small and increasingly beleaguered contingent of the Dominion's forces trapped in the alpha quadrant.