It's not evil, but it undermines the effect of the GPL. The GPL ensures that any product using your GPL code cannot become proprietary - it has to be licensed under a compatible license, compatibility depending on how it uses your code. What the 5-year expiry date would do is allow, say, a proprietary fork of Linux 2.6.30 to emerge 5 years from now. Later versions would have later effective expiry dates, but the point is that the product itself could have proprietary derivatives, exactly what the GPL prevents.
The reason to prevent proprietary derivatives is to ensure that enhancements, extensions, etc. are available to users of the original code. This is most of the reason Linux is so ridiculously powerful today, having been extended by many commercial institutions, with most of the changes being merged into the mainline for all to share.
However, it is clear that a forked Linux would still not be distributable in countries without the 5-year expiry, so the actual influence of the fork would be very limited.