Rick Berman is to blame.
If I had a penny for every time I heard that...
Rick Berman is to blame for giving us many years of enjoyable Star Trek after Gene Rodenberry died. Yes, it was imperfect - just as Star Trek was imperfect in the years prior to him taking over
As for the OP and the Borg: "Invincible" enemies, as the Borg were portrayed as up until "Descent" (when Beverly, of all people, assisted by a group of newbie halfwits, effortlessly destroyed a Borg ship), lose their aura of invincibility when they're defeated time and time again. It was inevitable.
They were still cool villains, IMO. And they'll be back one day.