They don't even need to do much, just add a category for developers who want to have a free + single-IAP-to-unlock-the-whole-thing version, and let people search that category specifically. No need for Apple to enforce time limits or anything. The trouble now isn't that demos can't exist, but that it's hard to tell which IAP-bearing apps are just full version unlocks and which are whale-chasing bullshit.

For that matter, just segregating games with any "consumable" IAP from those without would probably help a lot.