In both cases, the contenders pose a huge threat to the reigning champs.
So, what do Microsoft and Blockbuster do to try to blunt these threats to their core business? They try to emulate them. Microsoft just announced yesterday that it will offer a free web-based version of its Office software some time next year (in an immediate response to Google's announcement last week regarding its Chrome OS challenge). And, Blockbuster Online -- which is a laggard behind Netflix in the nascent online distribution game -- just announced a major new deal with Samsung to bring its online movie services to Internet-connected consumer electronics devices (including TVs).
Who will win? In the long-run, both challengers have a real shot of making significant inroads (although, in the Microsoft/Google case, the "long-run" is much much longer, given Microsoft's dominant position on the desktop and everyone's long-time familiarity with the product).
Blockbuster is more immediately challenged in all respects. Case in point -- I just went to a Blockbuster store a few days back, only to find that it had been shuttered ...