Nerfing old content when new content comes out is a HUGE mistake.

Blizzard tried that model and they haven't posted a single net-positive quarter for sub growth/retention ever since. Not one. They can't keep up with the demand for content, and they have a LOT more money to dump into their MMO than SE does for theirs.

Do what XI and vanilla/TBC did for years: Build on your content. Don't make it obsolete every patch.