At $499, the i9-9900k is almost competing against the 12-core Threadripper 2920x ($649, 12-core/24-threads, 4.4GHz clock, 60 PCIe lanes, quad-memory channels).
I think most people will find more use out of +4 cores (granted, on a NUMA platform) than higher clocks. Cores for compiling code, rendering, video editing, etc. etc.
Pretty much only gamers want +Clock speeds, and more and more games actually use all-cores these days (Doom, Battlefield, etc. etc.)
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That's the thing. The i9-9900k isn't even a "high end chip" anymore. Its at best, "highest of the mid-range" since the HEDT category (AMD Threadripper, or Intel-X) has been invented.
Once you start getting into 8-cores/16 threads, I start to worry about dual-memory channels and 16x PCIe lanes + 4GB/s DMI to the Southbridge. Its getting harder and harder to "feed the beast". A more balanced HEDT system (like Threadripper's quad-memory channels + 60PCIe lanes) just makes more sense.