That would entirely depend on the availability of the GTX1080 and its price relative to the price and performance of cards that we don't have benchmarks for. Given the nearly doubled price of a GTX2080, at the moment, compared to the GTX1080 it would have to be double the performance to be worth it. But then it becomes a matter of how much GPU do you need, these are high end cards for high end monitors.
Typically, Intel's prices are somewhat fixed. But they have been messing up their SKUs of late. Since the i7-9700k represents an entirely new class of CPU (8 core/8 threads), that is again, very hard to say until people start testing them. I suspect the i9-9900k will offer similar performance to the i7-8700k for games. Given the price of the i9-9900k, it may make the i7-8700k a viable option in May. Not sure about motherboard compatibility yet. I assume the Z390 chipset will still support 8th gen chips.
A GTX1080 is a very decent card. I've had one since launch, but that was basically 2 years ago. I paid $650, you can get the same card today for $515, or the cheapest GTX1080 for $450. Pascal is out of manufacture, but there is a lot of over-production to get through. This whole new pricing scheme for the RTX cards is likely the result of keeping product differentiation so the old cards can still be sold. So again, 8 or 9 months from now they may be all gone, and the ones that remain command high prices, they usually do. My hope is for a 2000 series replacement in a price bracket down. Then you can spend the excess on a monitor.
If you have a fixed budget the optimal build will change from week to week due to sales and availability. If you have a flexible budget, then you can start to look at things in terms of goal achievement. That is when you start sorting parts choice by targets like 4K @ 60hz (Or 4K @ 120hz), 3440x1440 @ 100hz, 2560x1440 @ 144/165hz. Triple screen gaming, the really wide displays (essentially dual 16:9)