Second Life dynamically downloads assets, as the world's objects, avatars, buildings, etc. are made by the people who made it.
Which is why it's always downloads stuff.
Of course, you may have already know that. :smileytongue:
The client itself is only about approx 30MB compared to the over (i guessing) 100MB that the Unreal Engine demo has (it downloads all the assets in one go).
By the way, Flash isn't dying, as it's still supported by Adobe.
Why wouldn't they make Flash this powerful if it didn't?
Even if HTML5 replaces Flash, Flash would still be used (if not for web apps and games, then for animations).
For now, though, HTML5 is primitive compared to Flash. Why can't we support Flash now?
If HTML5 ever gets as advanced as Flash as well as having a C++ compiler for it like what Flash currently has, then we can consider supporting HTML5.
See this for myths about HTML5. One of it is about it killing Flash. Another one is "HTML5 won’t be ready till 2022." (quoted from the article).