Someone did actually make this suggestion before the proposals were put together. The problem is a gradually changing membership fee would be complex to implement.
Another factor behind this and other changes is the complexity of the existing BAA membership structure. There are currently 16 categories of membership, that split into 26 categories for administration purposes with the overseas rates. The configurations of these different membership types leads to a lot of behind the scenes complexity, both for staff and the IT systems. We get them working, but it has led to a variety of mistakes and complaints, as well as expense to get things fixed. This is not the primary reason behind this and other proposed changes, but the complexity of running the BAA is a consideration.