This is versus the current antenna design which uses liquid crystal polymer (LCP) antenna technology. The shift to a modified-PI structure means that it could potentially reduce production costs, which in turn will hopefully mean that we will either see reduced prices or see Apple maintain their prices, which honestly is getting a bit out of hand.
In addition to keeping costs down, the new antenna tech is also said to offer up similar performance which means that users won’t see a degradation despite the switch in technology, and that it could potentially help with indoor navigation as well, something that still isn’t quite as ubiquitous as we would like to see yet (imagine using your phone as a GPS to navigate a mall, hotel, or office building).
Take this with a grain of salt for now, but if you are planning on getting the 2019 iPhones, these are some of the potential improvements and changes that you might be able to expect.