While there are certainly people who would refuse to sell at any price (Zuckerberg was probably in this camp), I suspect the vast majority of people (even 100% ethical, moral, up-standing folks) have a price.
Would I sell my business for $100m tomorrow even if the whole thing got destroyed in the process? I sure would (though I'd use the money to start a new one ;-)).
So if Marco was paid enough, would he let big companies shut down Instapaper by acquiring it?