It's about the implementation of raytracing not the performance. Anyone who thinks the performance of the 2080 will get better in raytracing over time must be out their mind if you ask me. It can handle 1 ray per pixel! 1... It's not even enough to do anything to the graphics.
I say get the 1080ti and wait until better ray trace cards are out. Maybe when the raytracing performance is around 100-1000x better we can think about buying one for that.
No one is saying the existing cores will magically increase, but the implementation will get better over time and 2080 will be supported for a long time to come. Historically, these kinds of paradigm shifts have left other cards practically worthless.