I take a different angle. It's normal that at some point you think you can offer unlimited storage because you have an estimate of what it means in practice. Then if people really start to store huge amounts of data, your estimates were wrong and you change your mind. No big deal.
I take issue though with how this is connected with the current reduction of 15GB to 5GB. 15GB is 4000 times lower than the single problematic instance. It has no relation. Also with a factor 3 you're not going to change your business model in any way. It's a simple optimization. There one would start to think that if you commit to offering 15GB, don't change it shortly after it or whenever you feel like it because then I start to think you're just doing whatever you can get away with.