Believe it or not, pp was formulated specifically to allow players to eventually reach a point where gaining pp is effectively impossible if they are not constantly improving.
If you were to get a score on every single map with high enough star rating to give any pp after weighting, you would be guaranteed to have hit that point (depending on your exact skill at different patterns, you will probably reach that point before you've played every single map that could potentially give you pp).
This is so that pp represents your skill. It's designed to prevent farming score the way things were before pp was introduced. Back then, your best score on every map you played would add to your ranked score, which is what rank was based off. This meant that someone who plays a ton of maps could reach the same rank as someone who didn't play many maps, but was much better than them.
This weighting system is probably the most important part of pp, even though it's generally overlooked.