Arch: for the young'uns with some fire left in them that just discovered open source and want to stick it to M$ and show off in front of friends.
Debian: When those people grow up and start having to do actual work on their computers...
I went through that cycle over the last 25 years. Thought I was hot shit running Slackware on a ThinkPad 380 when all my friends were on Windows 98. Then I got better things to do than running configure scripts all day and tweaking the UI yet again.