As "angrycat" states, airlines that are not flying routes and allowing slots to lapse bring in NO INCOME (either from the pax or airline) for the airport involved.
So watchout for the airports to go hunting airlines who can and will.
The low-cost airlines will be the ones to fill-in the empty slots because the bigger/flag carrier ones are "downsizing".
[ 11 October 2001: Message edited by: dwlpl ]