I won't start a Mattus-style thread - clearly we have ideological differences about things that cannot be resolved in a thread - but I'll quickly say a few things:
Silverlight may be open, but it's "open" the same way that Win32 or PDF are open: the specification is still controlled by a single corporate entity, it isn't truly open the way HTML and the web is. Even if Microsoft gave ECMA even the trademark rights to Silverlight the future direction of the platform will be dictated by Microsoft and not the community as a whole.
By "accessible" I mean the same content works on a finger-input smartphone with a tiny screen, to a mouse-driven 30" 2560x1440 behemoth desktop. DPI-independent scaling is only part of the solution, a Silverlight application is still going to have tiny checkboxes, fixed-size textboxes, and scrollbars and the runtime is not in a position to re-flow or reformat a document for a different device the way it can for HTML/CSS. This is why the web wasn't built on PDF or HyperStack, even though both support hypertext. (of course, HTML is inherently accessible, but I won't argue this point because you can easily make inaccessible websites).
As for video: the other sites are pragmatists: not everyone has a HTML5-video capable browser, and only WebKit browsers are really capable of live streaming of adaptive video (which you can actually do without any third-party components or plugins, but only WebKit can do it right now). I think that eventually, 3-5 years from now, Flash for Video will be dead when HTML5 <video> DRM support takes off.
Thank you for the insight in SQL Injection and XSS - I never thought of applications that way.
I'll add that another barrier to Silverlight's web dominance is tooling: Silverlight files are essentially binary, you need special tools to make something. You cannot quickly make something like a "hello world" with simple tools like only a text editor in under 30 seconds. Instead you really need Blend or Visual Studio or one of Mono's tools - this is hardly something your 12 yearold tinkerer can experiment with (this is also why I think F12 tools should be more prominent in every web browser).
Similar arguments to the ones you're making could be applied to Java <applets> from the late-1990s, and look what happened to Java in the end.
(I also find it a bit ironic that myself, as an MS FTE is opposed to Silverlight, and you're the outside who thinks it's the best thing ever )