You only get so many recoveries per level. So, if you go 5MWD'ing your way through a dungeon, you run out of recoveries before you earn enough exp to get to level 2. They're a finite resource within a given level.
Yeah, but if you are 5MWD'ing your way through a dungeon, you are only wasting resources if you actually waste resource. If you cast your Fireball on a single Goblin, yeah, you will have issues. But if you use your resources responsibly, all that 5MWD'ing your way through a dungeon means is that you are going through 15 rooms in a single delve instead of holing up after 5 rooms while out of resources.
I do understand that if recoveries can be used in a very short period of time, 5MWD doesn't occur. The PCs just keep going and going and if they responsibly use up all of their resources and recoveries repeatedly, they are also gaining enough XP to level up and regain those recoveries. An entire large dungeon can be done in a few "in-game" hours (i.e. maybe all in a single day) because the PCs just kept going and they might be one or two or even more levels higher when they walk out.
But if recoveries take a long time, then I don't see how they address the 5MWD issue.