You know, we effectively had this back in 1995 or 1996 when Netscape would release betas. Each version would expire and require you to upgrade a few months afterwards. The Web was very different back then, but I think that this could possibly work for browsers like Safari where there aren't a bunch of folks with plugins. But breaking my favorite Firefox plug because the plugin creator was on vacation for six months and missed a browser upgrade would be not so cool.