It'll be one of those things where time will tell.

I have sympathy for those involved in encoding but one has to remember that disruptive technologies are called disruptive for a reason. If WebM takes off its going to cause someone pain during the process... :)

Since you edited I'll edit too:

Which of those devices will still be around and usable in 5 years? Which ones won't be replaced by shiny new hardware? Your timeframe is a bit different than Google's.