Frankly, what do you expect them to do? They are in it for the money. Operating systems in the narrowest sense (scheduler, memory manager plus file system) became commodities decades ago. Graphics libraries, networking, printing, video and audio player libraries, databases, etc. followed. Even DOS had a fullscreen text editor, in later versions.

With complete 2GHz systems costing $250 or so, they would have a hard time selling people "we worked hard for a year, and here it is: it does the same, but it has a slightly faster/more memory efficient/more robust kernel" (battery live drives developments here a bit, but that is about it)

So, they start adding features. They could have made APIs for features and allow third parties to plug into them, but I guess doing that would have been more of an effort than building a good one (Android has it easier there with its Java infrastructure), and it just isn't in their DNA to sell a Lego set or to focus on making money from their hardware.

Yes, this will hurt some third party software, but I can understand why they 'have' to do this (and no, I do not think they could have gotten away with adding a totally new set of original features instead; people would still have complained about the lack of a decent notification system)