ARCH is a bleeding-edge, rolling distro, its main contra consists in the fact that constant updates can easily "break" the system. If you know enough, however, you quickly recover your setup.
I use and love it, but I'd never say "ARCH is better than Slack", let alone "Slackware is dead". It's plain childish to state so.
The kernel is upgraded quite frequently on a rolling OS, for instance. This model seems to be the main prototype for the future of operating systems. You get that feeling that you're always on the edge of technology, which is also elusive. ARCH fora hugely increase in help requests after system upgrades are available, which don't often come with enhancements for everybody's computers.
Slackware releases, on the other hand, are only available when the system has proven rock-solid stability. Not everyone needs the latest updates in order to keep up with their long-established routines.
I tend to think that one only needs an updated system if any piece of software or hardware is not behaving or performing as desired.