X399 vs X299

Honestly, for strictly gaming, there's absolutely no need for 16 cores and 32 threads. Most games these days will only slightly benefit from having more than 4 cores, and you'll probably never see any benefit from having more than 8 cores for years, at which point the processor's per-thread performance will likely be obsolete. It's not in the best interest of game developers to make a game that performs poorly on 99% of computers, so they try to focus on making the game logic run well on the hardware people actually have, which tends to be quad-core processors. Because of that, most games will only make heavy use of a handful of threads, so the majority of cores in a 16 core processor would be sitting dormant, doing practically nothing during nearly all games. Threadripper's 16 cores and 32 threads can be great for tasks that can actually make use of as many threads as you have available, like video encoding and rendering, but for gaming, all those extra cores won't likely offer much benefit at all.

And since the majority of current games tend to benefit more from higher per-core performance, Intel's CPUs generally come out on top, due to their higher clock rates and slightly higher IPC, at least when it comes to higher-end parts in the $300+ range. This is more relevant for screens with refresh rates higher than 60Hz though, since a CPU really only needs to have the performance to push whatever refresh rate your monitor can display, and if it can't show more than 60 frames per second, it doesn't matter as much which processor you have. I don't believe you mentioned what screen you were going to be using, but practically all current 4K monitors top out at 60hz. From a CPU performance standpoint, you would likely get slightly better gaming performance from a consumer-oriented processor, like the just-released 6 core, 12 thread 8700k on the Z370 platform, than on either X399 or X299, but you would probably not be able to have all those SSDs.

And while AMD's motherboards will likely get supported with new processors for longer, that's only useful if the gaming performance of those processors will be competitive a few years down the line, and that's simply unknown at this time. They will undoubtedly improve over what's currently available, but it's possible that Intel's processors might improve more, in which case you might be inclined to get a new motherboard anyway.