Eh, that's not exactly how it works. You're confusing physical device resolution/size with the logical resolution that developers have to deal with. From a development perspective, with the exception that you have to make 1x, 2x, and (now) 3x bitmaps:
* iPad 2 = iPad Mini = iPad 3 = iPad 4 = iPad Air = iPad Mini Retina
* iPhone 4S is a "sort of" outlier. If you're using the auto-layout stuff that's available it "just works" on a 5* phone. The exception is apps that aren't widget-based, like games.
* iPhone 6 and 6+ seem to have different logical resolutions, so there's that. But again, auto-layout helps here.
The "different DPI" thing is irrelevant. Get the UI elements to have the same size proportionally and you're set.
That's actually primarily for phones. For tablets you're supposed to use sw<x>dp. So, add sw600dp and sw720dp to your list. Now we're talking about 8 logical resolutions to support. And the bigger issue, as another person pointed out, is the fact that a _lot_ of the new APIs aren't backported. So you end up having to target the lowest common denominator API version, which is almost certainly two years old.
Contrast that with iOS. The latest version adoption rates are something absurd like 90% within the first month. You can actually take advantage of those spiffy new APIs without having to worry about limiting your audience. The limited set of resolutions is icing on the cake... far less to worry about.
This is only true with regards to screen resolutions. From my experience, there are more issues caused by different hardware (making camera work with all devices require a lot of testing), and custom keyboards also cause problems. (Custom keyboards will affect iOS 8 as well, but we will see if it will cause problems. This depends a lot on API design.)
When it comes to Android fragmentation, at least personally, I believe it has more to do with hardware specs than screen resolution.
I remember at my old job, we had an issue with a game being shipped because a handful of phones wouldn't be able to get past the first screen after hitting play. IIRC the issue was around the processor (or something hardware related) not having the ability to render certain things from Unity. It affected about 5 phones, but the published deemed it big enough to not be shippable.
as a note, xxxhdpi is only used for launcher icons (on the nexus 5 and possibly others). 640dpi screens seems like it would only be plausible for VR headsets maybe.
* iPad 2 = iPad Mini = iPad 3 = iPad 4 = iPad Air = iPad Mini Retina
* iPhone 5 = iPhone 5C = iPhone 5S = iPod Touch 5th gen
* iPhone 4S is a "sort of" outlier. If you're using the auto-layout stuff that's available it "just works" on a 5* phone. The exception is apps that aren't widget-based, like games.
* iPhone 6 and 6+ seem to have different logical resolutions, so there's that. But again, auto-layout helps here.
The "different DPI" thing is irrelevant. Get the UI elements to have the same size proportionally and you're set.