What is magic potion?Frequently asked questions about magic potion:
What happens to granaries and roads when I have Magic Potion?Granaries and roads are not useful any more when you have unlimited food (with Magic Potion).
These buildings do not age when you have Magic Potion. When your Magic Potion expires you will get access to them again as if time had been frozen.
In the meantime, granaries and roads are nice ways to customize your world and so even if they are not useful with Magic Potion, they do add a lot of flavour to your world. And it is always very nice to see.