I don't think there is any reason for them to decrease caps other than making more money by rent-seeking.
They pay for a fixed unlimited pipe from the transit providers right? So, the only reason for them to have data caps is to manage usage so that all the customers can be provided with the speed they pay for.
However, demand for data isn't perfectly elastic, even if you double the caps you may only see a small increase in usage. Most people only use a fraction of the FUP they are provided with.