Considering the RTX 2080 only performs about the same as a GTX 1080 Ti on average, and costs $800+ at this time, I would not expect the price of the 1080 Ti to drop significantly any time soon. The 2080 also has RTX features, which might make it more desirable in the future, but as of now, no games support that functionality whatsoever, so I can't see all that many people dropping their 1080 Ti for a 20-series card.
What makes 2080 more attractive than 1080ti will be DLSS not so much about RT. If nvidia can be very agressive adding DLSS support into existing games 2080 will hold up much better against 1080ti in the future. Also i think both nvidia and board partner will try their best not to discount existing 1080ti as both company will want to retain the same profit margin before 20 series coming into the market.