But you forgot to mention in your second section:
What would laners gain if Smite wasn't a summoner spell? Especially your Skirmisher's Sabre, I could see that on mid lane or ADCs in particular. Scoot out of lane, take raptors/krugs/gromp. Congrats, you're now beating your lane opponent.
What would junglers lose if Smite wasn't a summoner spell? In theory, maybe nothing. Except now that it isn't a summoner spell, you can no longer secure a camp/objective. It becomes a fight for it, and it's no longer a skill matchup - rather, more of a randomness feature as to who so happens to last hit it, which randomness in a moba really isn't okay, look at all the outcry about crits lately - as to who can more accurately time their smite, unless you're saying the item has an active with smite attached to it.
When making an argument you can't look at just one side. You have to look at both, for everyone affected.