The problem is not how many apps are being developed but how good are the apps being developed. This is more problematic as it requires some assessment of quality as well as usefulness and profitability. How many of these developers are writing on genuinely new stuff and how many are just repackaging? How many are breaking new ground and how many are developing a front end for an existing retailer/manufacture/advertiser/coupon pusher/etc.
The 'early adopters' on any platform will, by definition, pay more for more apps of dubious quality: those who adopt expensive stuff doubly so. As the platform matures the early adopters move on and the easy money moves with them. Of course developers follow the money. Of course there are more developers of esoteric apps looking at newer platforms; it's a less crowded selling space.
As far as I can see this is not developers leaving sinking ships but critical mass having been achieved.