Not sure that this statement lines up as SWGE will bring more people to the park and yes, they will be there for SW, but the only exit for the land is going to be straight through TSL. The land is going to be hit hard with crowds much larger than what the park is currently seeing. Even if Slinky and Saucers aren't necessarily seen as main draws, the land is still new to most guests that will be coming for SWGE and MMRR will be new as well. That means if you are someone who traveled to WDW even as late as June 2018, there will be FIVE new attractions at DHS for you by the time SWGE opens, plus the new Lightning McQueen show.
DHS is going to be the park that everybody is going to want to go to (although it may eat into a day at Epcot or DAK), but the point is simple - I think lines will stay about where they are now (normally I see Saucers at 30-45 and Slinky at 90-120).