Cores are like wheelchairs. Its great to have 1. But you really need a few for maintainance reasons, backup, etc. But once you get past 4 they are all sitting there unused. 16? More cores has less and less advantage.
2 makes a huge difference. Esp if multithreaded. Adding 2 more cores gains maybe 25% more performance - on certain programs. Adding 2 more adds 10%. Adding 2 more adds 5%... Only if they are used. Mostly because almost everything you do only uses 1 core. At best, 2 or possibly 3 at a push if you use complex stuff like movie editing. So unless you are making a habit of running a bunch of programs at once that all utilize 100% CPU per thread, its all a bit OTT.
There are the odd exceptions. Like in solid 3D modeling. But again the graphics card has started to do much of the work. But very few people use that. Or can learn it...
But if they come free then... At least the benchmarks will use them all for 2 seconds.
Whats actually happened is the CPU development continues (it must or the companies go bust). And the software / operating systems we are all using has got less demanding since XP, or the same, not the other way around. And things like office or web browsing, movies, etc - all the stuff you do daily - hasnt got greedier for computing power since the days of XP either. So now all the ever faster PCs year on year, are massive overkill. The only real gains have been in SSD and fast drives. Because those things were already a problem in the days of XP. And again were the main bottleneck. Still are. Of curse 4 and now 8K graphics, and games ARE more demanding. So buying a Nvidia 1080 or whatever (about 600 dollars) is one place that is a required upgrade. If you game.