Time travel is 'risky', 'sometimes dangerous'?
The borg destroyed thousands of species, killed and assimilated TRILLIONS. It has cubes numbering in the millions, military technology far in advance of the federation.
It is THE military power in the galaxy - overwhelmingly superior to the federation. The federation has no realistic chance of ending it by any conventional means.
It makes time travel - and its attendant complications (any hypothetical, hand-waving, if-you-save-this-man-there-is-a-chance-he'll-become-Stalin adversary could not damage the federation more than the borg can - as in, utterly destroy it with ease; the dominion is a third rate bully by comparison; complacency - really?) look like the safest thing in the world.
Taking only decades - plus however long it takes to find their origin - for a realistic chance to end the borg is a true bargain.