Well by moving to HTML5 you'd loose more users then you gain (smartphones and tablets), secondly at least on smartphones people tend to accept that not everything on a webpage work.
The only thing that really matters here is video (and probably audio). But all other uses of flash will stay this way for the foreseeable future. (Not that I like that but that's unfortunately the reality).
The hype aside the market share of the iPad (compared to other web clients) is at 2.13%  which is almost irrelevant compared to windows based systems.