Why is this such an issue?  In terms of web development then as it currently stands Silverlight should be used in moderation like this site does for the video player.

As for application development then this is where Silverlight makes sense.  Also developers cannot really start using everything HTML 5 has to offer for atleast 5 years.  I like most developers still have to cater for IE6.

Microsoft have this time to tweak their strategy.  I'd also imagine building high performance applications/games with HTML 5 to be a nightmare.  Most the people on this site are used to writing in a compiled language for the many benefits you get.  HTML 5 has alot of catching up to do.  If HTML 5 does take off then i'm sure Microsoft will provide the tools to make working with it as easy as building Silverlight applications and upgrading your applications as simple as possible.