Do we really need a rule that wandering monsters dry up after an area is cleared?
If the guidelines for players are any good, they will talk about different approaches to designing a setting, and a scenario, and the different sorts of tools that can be used - including wandering monsters - and how these can be incorporated into the story. The question of whether or not wandering monsters dry up is just a footnote to that broader discussion of playstyles and GMing techniques.
I hope not. It only needs considering if large numbers of novice DMs really will assume that you keep churning monsters out infinitely just because the book doesn't say "Stop". I've personally never met or even heard of anyone who does that. I'd be happy for the rules to remind the DM that any change in an area's population density also changes the wandering monster frequency.