The most important reason for why Air Drake should give Tenacity
Ocean drake is very useful for or against poke compositions. Yet there are common team compositions currently that are not really affected by the dragon buffs and those teams would be the wombo combo and heavy CC compositions. Tenacity would be the change that would increase the priority of air drake and dragons as a whole for teams who want the CC reduction while incentivizing teams to deny a buff that could be detrimental to CC reliant teams.

cause of just all the health regen they get already with 0 mana
and then there are
That's quite a lot of champions who just get like half of the buff which most don't even care too much if they have it or not. Mostly cause that's their play style and they have ways to either regen back or aren't ment to stay in long, pick up kills and then leave. And almost every tank gets almost nothing from infernal because it doesn't grant health.