What happened to the Borg was neither series' "fault" exactly, it was inevitable given two conditions:
1.The Borg were supposed to be almost unstoppable
2. The Borg became extremely popular villains who drew a lot of fanbase interest and enthusiasm
see, you can't really have an almost unstoppable villain that you keep bringing back and then defeating. It just doesn't work. But they wanted to keep using the Borg because they were cool, so you can't have it both ways. Either use them sparingly and preserve the scariness, or keep using them for the ratings, but diminish their power as they are continually defeated.