Whether it's down to expiring offers or just the size of the market, i think it all boils down to the demand. Even if a huge subsidy exists, if the airline can make more money by operating a different route, they're likely to do that.
Everyone sees reverses, but it also means an opportunity of other airlines. The exiting airline will have stirred up some demand, meaning a new entrant to the route can capitalise on this + their existing market, making it slightly more popular and more likely to last.
Unless it's a heavily competed route, in which case it's purely competition and any excess will quickly be snapped up Take PMI for example, ZB left, now we have a daily Primera and extra LS services to plug the gap. Popular routes don't stay empty for long, they're all chasing the money