It is indeed a very good idea but I don't see this will get implemented anytime soon because,
1. You need to spend resources for building and upgrading warehouses and granaries. If you combine them you will have plenty of resources to spare and that's not helpful in prolonging the game. Soon everyone will have so many troops and start fighting head on. As you know Travian is a game of math and calculation utilising what you have to reach where you want to be
2. If both are combined the cost of upgrade will get increased and then there will be wastage of resources used to upgrade the building especially in 15c croppers or capitals where you will be building more granaries than warehouses.