This is pretty much absolutely wrong.
Once you beat Echo of Lilith the game is done (at least for that character).
Duriel farming is completely optional and unnecessary. The entire uber boss farm was added for the sole purpose of appeasing people that couldn’t grasp that they didn’t actually need uber uniques.
Plus, the Duriel farming process starts at level 75 not 100, or earlier if you move quickly to farming Living Steel. If you’re starting at 100 you’re just gimping any semblance of difficulty. Not that the process is very hard anyway but a level 100 character beating on a level 75 boss to get summoning materials is pretty silly.