In a sea of millions there are plenty of new mods who will step up and take the reigns. Some of the really small subs might die off when their primary mods leaves, but most people aren't in the really small subs or they wouldn't be small. If r/gaming staged a walk out tomorrow and forever, there would be a dozen people stepping up to fill those mod seats tomorrow. And Reddit would let them happily.
The problem is that Reddit mods don't really create content. They help in its aggregation, and this is not a unique skill-set which would prevent a new generation of mods from coming about. Even if they wrecked the place on the way out by deleting all the CSS that made the sub unique, Reddit would just restore it for the new tenants. There are seriously talented mods out there who writing the CSS and meticulously curating major subs for no pay, and there are a dozen more waiting for them to leave so that they can do the same thing. Enough people want power, even if its on the internet, to make Reddit really hard to kill.