I don't like IAPs but this blog post's argument misses a really big key point. To do a fair comparison between a real game such as Dungeon Keeper and its IAP version, you can't quote the GOG price for the real game. That's the price that arrived years after the game was fresh with healthy sales and years after the game already recouped all of its costs.

The price they should have quoted for the original Dungeon Keeper game is $49.99.