High Damage By Itself Can't Make a Champ Viable
The Darius rework really crystallizes something that I've been thinking about for awhile: damage by itself is largely irrelevant to a champion's viability. Super high damage champions with no mobility and limited or no CC can't be balanced. If they do enough damage to be viable in high level games then they pub stomp low level games to a ridiculous degree.
CC is valuable because it lets you punish enemy positioning mistakes or outplay someone.
Mobility is valuable because it lets you punish enemy positioning mistakes or outplay someone.
Damage never lets you outplay someone. It punishes someone who doesn't appreciate your damage, but higher level players almost always understand how much damage a particular champion can put out at a particular point. Damage is only effective against players who don't understand how much damage you can do. Anyone who knows you are going to kill them is just going to walk away from you, and without mobility or CC you generally can't do anything about it. If they have mobility or CC then you almost certainly can't do anything about it.
The other major factor is range. If you can do damage from a range that your opponent cannot, then you have an advantage. The only counter to this advantage is high mobility or CC. If you are more mobile than the enemy, then you can get into range to do damage as well. If you have CC, then you can use flash to get into range and prevent the enemy from getting out of range. If you are less mobile than the enemy, have less range, and have limited or no CC, you are basically not a threat. Welcome to every low mobility melee champion's (except Olaf) experience trying to kill Vayne or Jinx late game.
Notably, there are different types of ranged damage, and different types of CC. Of particular note, if your ranged damage or CC is a skill shot, then it creates an opportunity for a skillful interaction between players. Notably, though, this skillful interaction is not champion neutral. Champions with high mobility have a larger advantage in dodging skill shots. The only thing that neutralizes this advantage is point and click abilities. These abilities represent counters to high mobility champions, but are just as effective against champions without high mobility.
Ashe offers an interesting window into some of the problems low mobility champions have in League. Ashe is herself a low mobility champion, and gets blown up by any form of CC. A Leona or Blitz lane totally wrecks her for the most part. Ironically, though, Ashe herself punishes other low mobility champions incredibly hard. If you get hit by Ashe once, you are going to be perma-slowed and probably won't get away without flash. In fact, it is my understanding that Ashe's ability to kite is in fact what first prompted the need to introduce high mobility into League. This
Ashe isn't alone, however. Many champions follow this same pattern. A lot of mages similarly have low mobility, and have kits that punish other low mobility champions. Lux comes to mind as a champion that can punish low mobility champions, but herself suffers from her own low mobility.
League is a game in which some champions have high mobility while others do not. As more and more champions offer high mobility, and less and less champions offer point and click abilities, mobility becomes more and more important. For the most part, newly released champions with mobility skills in their kit see competitive play, while those without high mobility do not.
In this context, damage numbers by themself become less important. Nasus is the ultimate example of this. Although he can scale up to do incredible damage, against many teams in higher level play he will have a hard time connecting with any of that damage. The only thing that even lets him be viable is wither, an incredibly strong skill CC skill that most other immobile champions aren't going to have anything comparable to.
The challenge is going to be finding a way to ensure that low mobility champions still have a role in games in which most other champions are more mobile than them.