Sustain is actually a very unreliable "tank" stat, as it relies on nobody ccing the sustainer, but if someone cc's them they lose all tankiness and will probably die before even getting control back depending on who or what cc'd them.
Whereas in contrast things like armor, mr, and health are consistent tank stats as no matter how much cc they get hit with they will still tank as much as if they didn't get cc'd.
Lifesteal is not as broken as people think, as when this ability was on old Sion, he was considered a troll pick, and that was more sustain than anything else league has ever seen, doesn't sound too broken when someone with a 75 ad steroid, 100% attack speed and lifesteal steroid is a troll pick.
The real problem is ranged lifesteal, as since they kite and stay far enough away that even if they get cc'd they will not be in every enemy's range so they don't get killed instantly, whereas a melee champ will be in range of just about every champion on the enemy team so as soon as they get cc'd they are dead pretty much instantly.