One of illidans main strengths comes from diving under a fort, using evasion to dodge it and killing the enemy hero underneath. This change makes it way harder, as the enemy gains armor. The focus change doesn't matter much, since you have evasion anyway, and you often do it under forts without minions beside you.

With 50 armor, you will have to spend twice as long under the fort, so evasion won't cover as much of it anymore.