Well for one, newer GPUs are just around the corner. I don't see how not having 15% gaming perf when the GPU bottleneck is removed is a good counter argument. Pretty nonsensical really. Your argument essentially means why even bother with a 5800X? Why not go lower? The CPU isn't the bottleneck in your argument right? Well we aren't talking about lower. We're talking about 5900X vs 5800X vs 5800X3D.
People know GPUs move quickly, will piecemeal upgrade that part, and you pick the CPU you will likely get the best value out of. Clearly a gaming focused CPU is more relevant for people who only use their desktop for gaming (I'm of course ignoring Word, Excel, web browsing as that needs trivial processing).