I mean the cap doesn’t even do that. People could level a 2nd character or do out of dungeon activities. There has been a feature since Vanilla where Parents can regulate their childrens gametime by only allowing them to play for a specific amount of time. Players could choose to enable this themselves if they know they lack self-control.
However you are assuming that adding the dungeon cap will suddenly make people be healthy and go outside or sleep or something else. The fact of the matter is adding a dungeon cap won’t guarantee any of this to happen.