Just out of curiosity, why is iMessage so important anyway? I mean doesn't pretty much every phone plan have unlimited or close to unlimited texts? I can see it being important years ago when you either had a low limit on the number of texts you could make or actually paid per text.
Something I'm not getting here?
I said this a while ago, what if one day iMessage becomes to Apple what BBM was to Blackberry? People clinging onto an aging platform for one chat feature.
This is not an "Apple is doomed" forecast, but more to agree with your point that I don't see what the big fuss iMessage is. I have many many many (many x100) friends, coworkers, family that have iPhones. I transitioned and left iMessage and have been doing just fine SMS-ing or chatting with them on Hangouts.
I'll go a step further, speaking generally now:
I've learned over the course of switching and using both platforms that a lot of the perceived things that lock you into Apple's iPhone are really not that detrimental to lose. People say the exclusive apps. Shrug. I've lived. The Play Store has exclusive apps, too. And in my opinion, the Play Store is organized better, looks better, and is easier to find apps than the App Store. Oh and Google doesn't prevent me from downloading apps just cause I'm not on WiFi (I still find this ABSURD).
What else? Updates? Nexus devices update just fine. But also... whatever. My device works great even if there is new software from Google. Also, most serious OEMs are making updates quicker than ever before. Samsung is expecting to roll out 6.0 in January. Motorola and Sony have both been pretty good (didn't Motorola one time roll out an update faster than some Nexus devices? I think that's kind of sad on Google's part, but kudos to Motorola). LG, too. What's a few months for bugs/kinks to be worked out? Plus, Google updates their core apps through the Play Store anyway. I'm rocking the latest iOS 9 on my iPhone 5S, and it definitely lags here and there. Big whoop I got it the first day. And to be clear, I'm not arguing against sooner updates; sure, bring them on. I'd love for OEMs and carriers to make the update process more streamlined. This would be great. But I'm also saying because updates don't come on the first day, it's not a dealbreaker for me to have to put up with all the rigid-ness of iOS.
iMessage? Eh. It's great, but as you've said, nowadays, there are unlimited texting. There are other free and widely used social and general messaging apps that do the same thing. Hangouts especially is good with this stuff.
Ecosystem with Macs is often said, too. And this too I can attest is not a big deal. Me and my family are all macs. Every single computer is a mac in the home. They all communicate just fine with my Android devices.
I'll happily lose all these for a truly advanced mobile operating system (EDIT: And better and prettier hardware with more useful features!). Anyway, that's just me.