They'd also have to rebalance partial long rests because those restore half of your spell slots and hitpoints. Getting half of your resources back is already good enough. They'd also have to adjust how camp events trigger so players that play in that mode won't be missing out on story related content.
I personally think the best method to encourage the player to not spam long rests would have been to have an event system that triggers on long rests that progresses the state of the world and the frequency of how often these events trigger gets tightened the higher the game goes up in difficulty.
So for instance, if the player doesn't manage to resolve the grove situation after 2 long rests it will automatically get raided by Minthara. If they don't resolve the Hag questline after 3, then Mayrina gets kidnapped. It would also encourage players to rotate through their party or occasionally set up party compositions that are not dependent on long rests in order to handle a situation that would automatically pan out in an unfavorable way. A party consisting mostly out of martial characters can push further for longer because their extra attacks aren't dependent on a limited resource while preserving the spell slots of spellcasters for the big fights.
