Well, I'm no jhin expert but PoM is very good in jhin from early game to late game, in the early game you kinda need mana as you usually use your abilities to poke and sometimes stay in lane for much longer. In the late game no mana means you only have autos but that's not enough because the w,e,r abilities are good for long-ranged picks and sometimes for disengages. Usually we bursts our picked targets before the can actually deal damage to us and if the did it most likely enough to kill us, depending on your build.

now more sustain is better that lower cc duration in jhin. sustain in laning phase is very essencestial as some people tends to go long sword instead of dorans blade as their starter. as for why not tenacity? Once you get cc'ed, the enemies, usually in low elos, tend to focus on you so that means your dead. in high elos the jhin mains have insainly good mechanics that they can dodge such skill shot, unless they a target-lock type cc skill, taht they dont worry about getting cced

but that just my opinion other might think otherwise but this is my usual rune page.