The imprisonment of the Old Gods weakens them, if they get loose then they go back to alpha level nuke form. They have in the past exerted power outside their imprisonment to stunning degrees. There are cracks in the cages that they can whisper out. I think the largest flexing of their power was during the Stormrage book where an old God nurished the nightmare lord into a Nordrassil sized tree that came within a hairs distance of destroying the emerald dream and the living thingd in the real world.
Another note I would make is that in war its the victors who get to write the history books, for all we know the Titans -were- usurpers and the Old Gods were not any different than any other pantheon (with various members being or representing "good" and "bad", usually more "good ones than bad") and that their death and imprisonment is what sent them insane in the first place. Its entirely possible that Elune herself represents a 'good' Old God. All of our historical lore tends to come directly from Titan based discoveries.