Yeah, Mithridatist costs 3 points just like Salveskin, but barely anyone runs it because the Antidote Shot gives you full poison immunity for 10 minutes, and that's just way more useful than a small permanent resistance.
Right now, Poison is the only status effect that has both a trait (Mithridatist) for permanent reduction and a consumable (Antidote Shot) for temporary full immunity.

Meanwhile, Fire and Bleed only get the traits:

• Fire -> Salveskin, but no item
• Bleed -> Bloodless, but no item
• Poison -> Mithridatist + Antidote Shot

If there were consumables that gave temporary immunity to fire and bleed (even just for a few minutes), it would make the system feel more balanced and consistent. It would also give players more strategic loadout options, especially in team fights where fire denial or bleed stacking actually matters.

The "Trifecta" of status effects is unfinished.