Great question. IAP, if jailbreak proof, should sure help everybody's sales.
And, IAP even simplifies the 'Free' version plus 'Paid' version concept.
Now you only have to manage one app, and it's free to download, then you can sell them more content later.
But of course you need to implement IAP, which may not be easy.
And not all games can apply this model I assume.
[Edit] "many people that have hacked and downloaded my Apps" . . . well, you could feel good that people liked it enough to steel it!
