KDE 3.5.x really was peak desktop at the time, especially when all the K* apps were working as intended. As a Windows user discovering Slackware 10.2 back 2005, I was really blown away. KDE 4 just didn't feel the same.
I guess one cares less about DE's when my DE is a front for a terminal and a VM (my browser).
It's sad to see sidelining of KOffice, Konqueror (of course still exists), Amarok and other great apps.
A desktop environemnt is not just a WM + Panel + Decorations.
KDE3.5x has is own... well everything tied together.