Do you really think Rylai's having 5 more Ability Power will cause it to be as strong as it was last season? Alternatively it could be reduced to 60 Ability Power and get more Health. Either way, Rylai's users will be happier.
And if Zhonia's was buffed, Mages who rush it would have Mana issues and would have to go back more often unless they get Lost Chapter first which takes care of most of their Mana issues. So Lost Chapter's passive would have to be nerfed. Now you have the options: Get a lower cost, less effective Morellonomicon-1 first and not have as many Mana issues, or get Zhonia's first and die less often but have to go back more often which reduces your map pressure and gives your Assassin opponent more freedom in roaming, farming or pushing. You can still get Lost Chapter first, but it won't be the same. If your opponent is someone like Vladimir you instead get Morellonomicon-2 first which isn't as powerful as option 1 but offers Grievous Wounds.
Plus, the old Zhonia's existed for a long time and Assasins were still powerful. I doubt they'll fall from grace just because their opponent has 10 more Ability Power. They have lots of strengths after all. And if Morellonomicon was nerfed to 80 and Rylai's buffed by 5 Mages will still have 5 less Ability Power.