It doesn't if you have them be the main enemy of the series and have them lose again and again.
Of course, if the Borg hadn't been so stupidly overpowered in the first place it would've been more sustainable.
If I'd created them, I'd have The Borg be actually weaker than the Feds, but their strength is in their regenerative powers and how they keep coming back. So in a straight up battle against a few Borg ships there's not TOO much danger but if you don't do something about the infestation they grow in number to the point their numbers make up for lack of individual power.
Like the swarm ships from Star Trek Beyond. Powerful in number, weak on their own, and if you find a way to disrupt the Swarm synchronization they're no threat at all.
They needed some excuse to depower the Borg for Voyager, if they were going to be their main enemy for 7 years.