Binding APIs are some work but not impossible for OSS community to replicate if the CoreCLR and friends provided a portable platform with liberal licensing - so even if Xamarin disappeared it could be replicated - but I see no reason for them to shut it down.
And the third part of Xamarin is tooling which they can roll up into their own product offering.
Basically acquihire is a good scenario in this case because Microsoft is already moving towards these areas, Xamarin brings in talent and a bridge with working tech to provide transition.
i think it's confirmation bias you don't really think of all the many successful acquisitions only the bad ones stand out.
"OneNote started as an email exchange between myself and Steven Sinofsky, the Senior VP for Office, Nov 27, 2000."
http://blogs.msdn.com/b/chris_pratley/archive/2004/01/30/one...
Also, I'm personally not happy about this. I think people on HN should know better. Besides a couple of black swans being the exceptions that confirm the rule, acquisitions is how projects die. This is because acquisitions are either defensive or acquihires. And even when done out of a genuine desire for progress, big companies end up choking these acquisitions to death, after all they weren't capable of such progress in-house, with the project members moving on after one or two years when their contract expires.
But hey, people were excited about Nokia as well.