Alle the catch up I see for android is that they will make it a more closed system .Modern android phones do not have excellent battery lives. Some of the phablets like the note 5 have fairly ok battery life but none are as good as the iPhone 6 Plus. Also I wouldn't call 1080p poor. It's fine enough. Most people are still using 1080p TVs. 720p on the 6S is Rubbish though. However maybe on such a small phone it doesn't matter. I don't know the 6S just doesn't appeal to me for several reasons (too small, poor battery life, not 1080p, no OIS).
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I agree with most of what you've said however don't you think that by 2018 android would have moved forward? They are not going to stand still and wait for Apple to 'catch up'. Anyhow having said that for all of androids technical superiority I'd still pick an iPhone any day of the week. There are too many compromises with android devices and a few impressive specs on a sheet of paper won't compensate for all the flaws.
They should do something with the update system, but that is more complex, because android manufactures always add their own look and apps.
The only difference Google could make is to market their nexus line more agressive. Or that all android manufacturers will use stock android.
Perhaps Google should produce their own phones , Google reference or something like that.
But if android becomes a more closed system, a lot of users won' t like it. They are always complaining about iOS being too limited.