If they have the right story, they should go for it. Why save the Borg for a tomorrow that may never come?
The problem is that if TOS starts using the heavy hitters of TNG instead of its own, there won't be much for a TNG reboot to play with for their own films.
It's a massive stretch at this point to assume that a TNG reboot will ever happen, so it makes no sense to restrict your story possibilities to "save it for the TNG reboot."
Besides, it would make a certain amount of sense to plot the TNG reboot literally a generation after Kirk's time -- say, if Picard actually answers to Admiral Kirk in the chain of command -- which case the Borg would be a persistent threat that continues to threaten the Federation, not a blast from the past that is was mysteriously dormant for a hundred years.