Making tanks tankier is NOT the solution to extending teamfights.
Well it IS a solution... to make teamfights REALLY long... REALLY REALLY long.
The issue in teamfights isnt tanks getting blown up, sure if 5 people focus you, youll die pretty quick, but tanks are supposed to be able to be killed if focused.
If tank resistances were buffed, then every teamfight would become a wet noodle slapping fight until the carries respawn.
No the issue lies in carries (which includes ALL carries Mages included) doing damage to eachother. A solution to extending teamfights is to increase carry resistances and/or lower damage to eachother.