Tessellation needs to be in the subpixel range throughout the entire screen before it can successfully and completely replace AA.
Tessellation can never replace AA.
You need AA to properly filter subpixels.
Quite simply, tessellation deals with geometry, AA deals with jagged edges on screen. You will ALWAYS have jagged edges if you don't do any kind of supersampling and filtering (aka AA), no matter how much geometric detail you have.
