Technically speaking, Adesanya doesn't need to do anything. He is the incumbent champ and is favored by the promotion. He has used that favor twice now to help elevate (arguably undeserving albeit dangerous) contenders into a position wherein they could challenge him for the title. Pereira won, Adesanya got an instant rematch (which I don't personally agree with, but is a common occurrence and is pretty standard precedent with longtime-defending/dominant champs), and then beat Pereira to reclaim his title. Now he (and presumably Dana judging solely by post-fight talk) have decided Pereira gets no more favors at 185 and it's a done deal. You are free to cry foul from the sidelines about how it's 1-1 or to bring up contests from another sport, but there is there is no moral or contractual obligation for Izzy to push for Pereira to fight him yet again.
You can say it's wrong or underhanded or a sign of cowardice or what have you and maybe you know what? Maybe you're right and Adesanya wakes up in bed every night in a cold sweat from night terrors about the idea of facing Poatan again. It's entirely possible. But, like I said, my sympathy for Alex is pretty limited. The guy led a charmed path to the title and got a world championship out of it. The fact that he doesn't get his precious instant rematch and would have to be forced to pick between moving up to 205 now or sticking around to fight a Top 5 guy or two and earn said rematch garners exceedingly little sympathy from me. I wish more ex-champs were willing to do what Whittaker did after losing to Izzy the first time, honestly.