Great post @geekgirl. I really enjoyed the content and i will try to summaries my comment.
Companies, especially big tend, always tend to move towards more and more monopoly and centralization. Perhaps, it is because that's how they see expanding business and creating more revenue streams.
From this highlighted context above, it is easier to picture the factor behind it.
This companies that still finds or channel more of their strength towards centralization are watching closely at the interest of their customers or users. Their customers which are the majority users of internet are still very interested in web2 platforms and are yet to consider a decentralized system. So this companies still sees opportunity to generate income from what most of their customers value.
Google and Apple
I don't see this different companies overtaking the other for the now but maybe it might happen in the future. Just like you stated "So, as long as both operating systems exist there will have to be two separate apps that do the same exact thing".
What Hive does can easily be copied
I would love to call hive a genuine social that was built base on the interest of its users. The idea of copying hive can't be exact because most companies still sees that very big opportunity of making money from centralization. If copying hive was that easy, then steem service would have be better and very competitive.
I see decentralization as something that maintains an order even without the aid of a middleman, which steem lacks.