The idea that people don't extend their vital survival spells is pretty ludicrous to me if they're expecting to be ganked. The warmage also has to get the initiative which slips easier since they can't timestop to position themselves, their haste is through potions or has to come from someone else, same with invis, the list goes on.
Wizard's just not getting nerfed because of warmage's addition. If anything, EO will just change the way this interacts with spells that deal damage more than once, maybe halving it or something if he sees it as necessary. Ultimately, you don't have to be completely immune to all damage to survive a gank. Even a misted magic acid fog can be escaped in a single round of normal movement if only you are immune to slows, which, if you're worried about warmage high enough level to maximise this spell, you probably are almost all the time or you have potions handy if he spooks you. If you're on equal terms and you want an equal chance to escape or fight back, this is one of the least one-sided fights you could pick.
edit: just to expand on the point above - yes, everyone can get one save to the point of immunity to most lower circle spells, but not everyone can get two, and very few classes can be immune to all three saves simultaneously. If you know who you're ganking, you probably know their class if you have a solid reason to kill them, so you know what they're weak to even when they are buffed.