Longer development time means less top level games in the same timeframe. That's an absolute boon to first party content. Less competition translates to higher average sales. This is why I'm convinced that Nintendo does not want "full" third-party support. Microsoft missed a golden opportunity by not being ready for this generation.
Sony's strategy to release throughout the year rather than focusing on the holidays, which largely take care of themselves with third-party offerings, is also beneficial. People can pretend that only the holiday season matters but releasing high-profile content when the competition level is low not only increases unit sales, it buoys the entire platform.