Look. I have no doubts that it gives lane sustain and I'm saying nothing to the contrary.
But that is not what you're saying.
You were comparing it to spell vamp and the areas where it's compared to spell vamp, it's strictly inferior. The amount is tied to spell cost so it does not scale where spell vamp scales naturally through the game and it has a cap of 25 health where spell vamp isn't capped and has no problems exceeding 25 health stolen.
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It sure is a lower amount of health, but it gives an extreme reliability.
No, it's a far lower amount of health and it is "reliable" in the same way spell vamp and any other passive, certainly not to any extreme extent
To heal just 15 health, the ability needs to have a mana cost of 100 mana, an ultimate level mana cost. For a
to outpace the health gained, you only need to hit a minion with an ability that does more than 125 damage or, for how healing off minions is usually done, hit the wave with an ability that does more than 62 damage. The times where the proposed catalyst would be comparable to the Revolver is where the abilities do less than 125 damage which is like Level 4 and before, where the champion won't even have a catalyst yet.
And that's if the abilities cost more than 100 mana; a champion whose offensive ability costs 100 mana or more yet does less than 125 damage is not a scenario that occurs in this game.
Sorry, but your proposal, the idea that "the new Catalyst is Spell Vamp but better" is mathematically false. It's worse.