1. That's not an iOS deficiency. The SMS/MMS app in iOS has always been better than practically anything on Android. How long did it take Android to get Group MMS support, while iOS and even Windows Phone has had this for years? The need to swap out the default SMS app on iOS is just not there for most people. You don't gain anything by doing that.
Contrarily, you lose a ton of functionality on Android by going from the (i.e.) Samsung SMS/MMS app to Google Hangouts. There is, literally, nothing to gain but a different user interface - and in some cases it will force you to use multiple applications to do what one could do before the switch. The benefits of having the threads in the Hangouts app is simply not worth using Hangouts in lieu of the Messaging app. The only way I'd use this in Hangouts is simply to be able to view and switch when I only have to send text (only) messages really quickly here and there. For heavy texters and MMSers, it's almost worthless.
This is not a deficiency on iOS. That's a policy. The reason why Apple (or Microsoft, for that matter - except with Camera apps now) do not allow swapping out the default apps is because they focus on providing a decent experience with the core apps on the platform. You're free to use an alternative, where possible, however...
Not everyone wants to, or even cares about swapping out apps. Some people get the phone they get based on what functionality is offered in the stock software so calling it a deficiency IS tunnel visioned and a misnomer.
2. There are complete apps like YouTube Camera that weren't even released for iOS. The Gmail and Maps apps got updated on iOS months before Android. Complete components like Google Now went to iOS while skipping even ICS devices. There are tons of examples.
3. Google+ only matters to people who want to be on Google+. Auto-Upload is not an Android feature, it's a Google+ feature in an app pre-loaded on Android devices. It's bloatware, basically. No different than Verizon pre-loading VZ Navigator on their devices... The fact that Google developed it doesn't change that. It isn't the same as SkyDrive auto-upload on Windows Phone, or Photo Streams on iOS, it's a feature tied to Google's own Facebook-clone. It's more analogous to the Photo Sync in Facebook which is equally useless to people who do not want to be on Facebook. Not comparable. I'm not sure I even mentioned Google+ features. I'm talking about core Google Services, not the ones dependent on their Social Network (which Hangouts is, so no iOS people I know will dare to use that app).
The overall core Google Experience, if you want the fastest app updates, and features at a faster pace that aren't tied to specific Android versions (that your phone may never receive) is better on iOS. There is literally NO doubt of that.
As for Google Search. Google now pulls a bunch of info from Wikipedia, while S Voice and Siri pulls a ton of information from Wolfram Alpha. That lets you know which one I tend to use...
4. Android was designed to be modular to allow OEMs to customize it otherwise they would get frustrated as most did with Windows Phone OEMs. 100 OEMs will not launch the same phones running the same stock software with only marginal hardware differences. The OS was designed that way. That is why Google has made it even easier to switch between Launchers and Messaging apps.
Right now, Google is taking a bit more control over the platform by superceding (and abandoning) AOSP components in favor of proprietary GApps.
Downloads do not mean users. Path, for example, has WAY more downloads on just Google Play than the amount of active users it has across all platforms it supports.
3rd party Launchers and Messaging apps (I've tried quite a few) are usually overly complicated, sometimes quite ugly, and overloaded with settings. They serve no purpose but to confuse people who are not quite tech literate - some of them even require plug-ins to accomplish things the stock apps can do with no such intermediary steps... This is why I stopped recommending them to people and it's also why I stopped purchasing devices based on theoretical functionality from 3rd party devices. I purchase based on what the device can do, as I hate installing redundant apps on my phone, anyways. This is why I never have (and probably never will) buy a Nexus. The functionality is always way behind a Sense or TouchWiz device as Google offers only the Least-Common Denominator feature-wise in their stock build (and I personally think it looks like poo, but that's is certainly personal).
No, I don't use an iOS device (4" is way too small for me), but it will be interesting to see what happens if Apple launches a 5" iPhone.
This isn't just a Google issue, either. Microsoft has similar support disparity between the updates to core components on their own smartphone OS vs what they launch for iOS (and to a lesser extent, Android) as well...