Or the other AMD fan argument for "moar cores", i.e. "future proofing". With a lot more cpu power in the next gen consoles, the 10900k could well be faster than the 9900k/10700k at some time in the future.Unless we use the same argument that AMD crowd have been using for a while: more cores let you run more things in the background, stream or whatever.
So if we say that games can use 8 cores now (I don't know, honestly), you get 2 extra cores for whatever else there's going on.
Some will choose Intel for the IGP. Some for other specific advantages (some things just work better on Intel).
In parallel loads 3900X is better.
In sequential (single-thread) tasks 10900K wins.
In a real-world, mixed use - it really depends what you're using.
There's no such thing as lower or higher parallel software. If a program uses 8 out of 10 cores, it means there are 8 single-thread tasks running.
An algorithm is called parallel when it's able to divide a problem into many separate elements that can be run independently. "Many" usually means going down to data granularity.
So if I have 2 numerical vectors with 10^6 elements and I add them, it can be split into 10^6 separate processes.