For most people the 3D is an amusing toy, not really useful. I'd be happy without it frankly.
OSM doesn't do elevation
See 3D above
OSM doesn't do geocoding
They have data for searching on street addresses and towns, and the data is available separately for postcodes etc (TomTom for example will have sold a solution for this to Apple, so they could have gone down that route). Google has this down, as you'd expect given their search expertise, and Apple would have done better to pay them at least for access to this service if nothing else as it's a complicated area.
OSM doesn't have a cleaned up dataset of Points of Interest
Yelp does, and that's who Apple are using for POIs
One other point you didn't mention is that OSM doesn't do satellite data (their stuff is from Yahoo isn't it?), so that is a big area Apple would have had to sort out from another provider.
While you're right that there is no one-stop shop for mapping services, and it's a really complex area (i.e. just choosing OSM would not solve all these probs, as you point out) I do think Apple have missed a chance to form a symbiotic relationship with OSM and enhance the quality of their maps with an open process rather than relying on the often out of date commercial data available and having to either fork the commercial data from this point on, or merge changes. Dealing with all these fixes will suck up a lot of time they could spend on improving the app itself.