Is ult mana costs slowly being standardized to 100 mana "bad game design"?

Me and a friend got into an argument. He looked at the changes to Talon's mana costs and says that Riot's trend of slowly standardizing most champions (I say most, because obviously champs like Leblanc, Udyr, Karma, etc. are different) mana cost to 100 mana is bad game design. He says that it will give an unfair advantage to champions with higher innate mana pools or champions that build mana in their builds, and that those champions with more mana will outperform champions with less mana.

His example was Karthus vs. Lux. He claims that if Lux and Karthus were to have the same mana values for their ults, that Lux is going to have a strict advantage over Karthus because she has an ult that costs the same amount of mana but is on a lower cooldown.

I tried to explain that it's silly to say that a champion is better than other champions just because they have more innate mana, or because they build more mana items, or because their ult has a lower cooldown. I tried to explain that it's how their entire kit works together that ends up determining a champions strength. I pointed out that yes, Lux can use her ultimate more often, but it's a skillshot that is difficult to land without first landing a binding, which is also a skillshot and another part of her kit, and that all of her abilities are skillshots tied to high mana costs and long cooldowns, which is very opposite of Karthus. I tried to explain that different abilities do different things in different situations, and you can't say something is strictly better than others or that champions will outperform just because they have lower cooldowns and/or higher mana.

Nobody probably cares but just in case someone actually takes the time to read this, I'd just like to get an outside party's opinion.