That shows the risks of vendor lock in. Although I don't believe MS will replace .NET with html5/js in any regard it shows pretty good why you shouldn't commit too much to vendor specific/centric technologies.
Based on that logic no one should be building iPhone or iPad apps. You have to commit to something at some point. The odds of a company ditching a product they have invested hugely into is probably the same as the odds of an open technology or framework simply becoming unpopular.
You can commit to something, for a time, without pushing everything else out of your brain. I spent quite some time doing iOS development, and it would certainly not be the end of the world if the entire toolchain changed tomorrow.