I know what your saying you can do this with browsers to a point, you can notify the user that their browser is out of date using user agent checks but ultimately in both solutions its up to the user to do the update, of course with the Flash update tool it provides the user a clean easy way to update the player. You don't have to tell them to go to their browser maker's website and download the latest version which will likely look very different from the version they currently know and use. Which for some like many members of my family changing their browser say for example from Firefox 3.6 to Firefox 4 was a big change, even IE8 to IE9 was a big change and confuses non-technical users. With the Flash player as far as the user is concerned all that changed is some number, it doesn't change how it looks or how it works to them it just works. So telling a user they HAVE to upgrade IE8 to IE9 to use your website is not a good idea, and for some of the IE8 users IE9 is not even available to them so they are stuck changing browsers which can be even more jaring than just a update.