There are some disadvantages that you should consider when examining whether a mattress topper can serve as your mattress.
Firstly, if you have already owned or purchased a new memory foam topper which is not very thick then you may not have enough foam to create a comfortable base. Anything less than 5cm is likely too small. Even though foam is designed to be soft and comfortable, if it is too thin then you will feel the bed base underneath or even worse the floor if you don't use a bed base. Pillows are a good example of this as often they become lumpy and uncomfortable when they do not have the right support underneath them. Most mattresses these days have high density foam (HDF) to provide a firm base and then layer on softer foam on top for comfort and support.
Another disadvantage is that you may find it difficult to clean if your mattress topper gets dirty. Mattress toppers, depending on what you buy, often have a casing to protect the foam. They aren't intended to be the only thing between you and your bedding and if you are sleeping on the floor then you might be more prone to dust mites getting into the foam. Once they are in the foam, they are notoriously hard to get rid of. Here's some related reading on washing mattress toppers. This can be overcome by having a bed base that's off the floor.