There are many reasons for choosing a non edible celebration cake. 
I have for many years been thinking of a great way to provide a celebration cake for my disabled daughter as she has an unsafe swallow and fed by tube. 
I have tried making cakes from socks and clothes, making cakes from lollies so that she has some enjoyment from them. 

However, her favourite by far is a dummy cake. 
A cake that looks like any other celebration cake, but can be kept and not eaten. 

For these I use a polystyrene base, covered and decorated with sugar paste in the same way that you would a standard birthday cake. 
The great thing about these is that they will pretty much last for as long as you want them to, providing they are stored correctly. 
Here are a few examples of dummy cakes that I have created.