The idea is interesting, and I would like it, but people would end up not having to buy potions, and champions like Rek'sai would get weakened a lot because her passive would become less relevant, and the Sustain Smite would take over the smite-item meta like the Warding Smite in seasons 6 and 7(might be a bit off with the seasons).
Overall, I would like the smite, especially on junglers like
, who I've had trouble with sustain on, but I feel like it would overpower the meta.
Maybe a similar smite would work, such as a mana-giving smite, but this particularly would probably overpower the jungle meta.
Good idea, though. :)