Samsung provides minimum 2 major OS upgrades. Samsung On Max got only one. So it will get the Pie upgrade. It does not matter if the processor is mediatek or not. It depends on Hardware compatibility. But because of Mediatek processor the upgrade will come to the device late. But you don't worry. Samsung Galaxy On Max will get the Pie upgrade in this year.
I told you processor does not matter. It depends on Hardware compatibility. I called Samsung Customer Care. They told it. They also said that every midrange devices get minimum 2 major OS upgrades if it came with Naugat out of the box, then it will get the Pie upgrade. But because of mediatek processor the device will get the upgrade late.