You don't get fat stacks in the first place without being frugal, though it's not solely down to price.
The Z390 board range is stated for early next year. Why bother with a Z370 at the point in time when the new boards are literally less than 2 months away.
As for For dropping the 1070 for a 1080Ti, why not make it last until later into next year and scoop up the Volta flagship card for the same price of buying a 1080Ti now?
Price inflation only really effects RAM for that build since coin mining has had largely no effect on 1080/1080Ti prices, but it's still something to consider.
Volta is no longer the next card branding for consumer market and itís call Ampere. The 1080 Ti just released in May so the next x80 Ti probably wonít come out till Fall 2019 if we follow the same release in schedule. Going from a 1070 to 1080 Ti is well worth the jump which I also did recently.
I can see where you're coming from but there's always something better just around the corner. Why buy the flagship Volta card next year, when you can buy the flagship Volta 2 card for the same price just 8 months later or something? I've seen people delaying their builds indefinitely because of this way of thinking.
If you have the money, and you want the top of the line, then go for it, I'd say. Doesn't matter when you step in, there's always something better coming.