While it would seem reasonable, if that results in a significant drop in in-app purchases Apple will no doubt raise other fees to compensate. For example, only allowing free hosting for apps with no purchases, in app or otherwise, and charging to host those that have purchases per download. They could credit that against in-app purchases; but it still would mean developers would have to pay upfront without knowing if they will recoup the costs in purchases. At some point, if less than x% buy something they will lose money.
The current system significantly reduces the costs and risks of bringing an app to market since the only significant costs are development; if distribution becomes an additional upfront cost that will impact a developer's cash flow and ability to turn a profit. In addition to the costs of a new app, a developer may find it unprofitable to leave an app up and update it, since every download would potentially cost money with no return.
Are Apple's fees unreasonable is a valid discussion; but potentially enabling developers to avoid paying any fee for an in-app purchase opens up another can of worms. Big developers could no doubt survive but smaller ones may feel a financial squeeze.