First, if I have my java browser plugin installed, I don't expect my browser to offer me to install my java browser plugin. It may well be a malicious site tricking me into something, who knows.

Second, this does not solve all the other myriad use cases that do not involve the browser, like stand-alone java programs. What if my java application does not run anymore after switching to a different implementation? I see my java package still installed and 'java' on the command line still works. That's some nice fun cases for "It worked yesterday and I haven't done anything in the meantime" admin calls.