Jim Dalrymple, in his review of the 13-inch MacBook Air, hits on exactly why I think the 15-inch MacBook Pro will be replaced by a powerful 15-inch MacBook Air:

With the release of the latest 13-inch MacBook [Air], Apple has once again reduced the number of factors users have to consider when purchasing a laptop computer.

There was a time when you went shopping for a laptop that you would have to consider all the things you wanted to do with the machine and eliminate models based on those criteria. Things like having enough power to record music or process a mix are a thing of the past.

All of Appleā€™s laptops are powerful enough to do all of those things these days. The only real consideration left is the screen size you want.

Screen Size Matters