Hi,
I don't believe it would be very likely that no one would step up if the seat really was vacant.
Unfortunately I know of at least two instances where that has happened in the past few years, after repeated calls for volunteers when the in-situ chair was stepping down, or had not decided whether they would re-stand. One of those instances led to me becoming NCC-Services co-chair after I'd done my time in Routing. :) It varies between working groups too, I can also recall a couple of different working groups that have had multiple volunteers recently and then wondered how to select the best candidate. Several WGs have tried to encourage a composition of two old hands and one new to the scene, but it hasn't always been successful (that's not to say we shouldn't keep on trying). I think it's fair to say that most, if not all, existing WG chairs and co-chairs would welcome some new volunteers and indeed advice on how to encourage people to stand, but the fame and glory may occasionally be over-stated. Cheers, Rob