Also (and there are many other arguments) it is not proper to take a single sentence and not consider it in the context of the full analysis of pleasure as to whether it has a limit, which is what Plato had argued in Philebus. The main reason to consider the total removal of all pain to be the limit of pleasure of pleasure is that unless you can logically define a limit for pleasure, pleasure can always be made better by adding more.
The citations from Plato and Seneca that document this argument about pleasure from the opposing side are in the "Full Cup" article here on the site.