While initially, I thought, "it's 4 openspaces being converted to one sim for the same money", it struck me that most of those who need to convert probably haven't had time to do so yet. They would need to coordinate it with the people using the four openspaces.
Also, there is no pressing reason to drop an openspace until the next billing; you might as well hang on to it and perhaps see if you can sell it to recoup a little. And some will likely hang on to them until the price increases kick in.
If this is true, the shrinkage so far represents only those who have thrown down the towel immediately in sheer disgust and protest, which would make the numbers pretty grim.