X299 is paired with LGA 2011, an interface that supports 44 CPU PCIe lanes and four memory channels. Z370 is paired with LGA 1151, which has only 16 CPU PCIe lanes and two memory channels.
So who would buy X299 after Z370 is out? Someone who needs more than 16 CPU lanes or more than two memory channels.
Kaby Lake X is a 16-lane processor that, when placed on an X299 platform, cripples half its features. It was always a bad idea for most users, but there are a few elite overclockers who love it just for overclocking.