I don't really see that. I mean even with the fastest DDR4 I only see large gains in synthetic benchmarks that focus on memory, like Sisoft Sandra and others. Real-world performance isn't there.
But you are definitely right about the platform (or chipset) being the best reason to upgrade from say, Z97 to Z370. The support for x4 NVMe drives natively (Z97 supports NVMe M.2 drives natively, but only at 10Gb/s speeds, AKA 1200MB/s) is a really big deal since these drives are so popular now. So I'd say NVMe support along with some other improvements in the chipset are the biggest reasons to upgrade. DDR4 is nice but not a game changer in my opinion.
I also do have to admit that the 8700K is a great CPU. If you can drop the cash for one and upgrade to the newest chipset, you will get a very decent upgrade. I was just comparing the lesser CPU's to Haswell's refresh, like the i5 8400. Let's also not forget that the 4790K all but matches the 6700K in performance, and is only very slightly slower than the 7700K. I was very surprised to see just how close these three CPU's perform. As far as the i5 8400 goes, I expected it to really do a lot better in heavily threaded apps against the 4790K but it just doesn't since the clockspeed is so low and it cannot be OC'ed easily. The 4790K hits 4.7Ghz pretty easily. I can hit over 5Ghz on mine if I crank up the vcore, although I prefer stability and life out of my hardware, so I run it at 4.7Ghz@1.9v myself.