I think when spock said "threatens the galaxy", he meant it in a more political sense.
With a fractured and weakened Romulan species, that invites the Klingons to get revenge. That can also invite many races that the Romulans have pissed off to attack.
The sheer chaos from that would be unmeasureable.
It could turn into 1 giant blood bath very easily.
Which could eventually draw the Federation right back into war so soon after the Dominion war.