Yes there are a lot of tasks that utilize only single core and single thread and that includes gaming (this is why the new Pentiums are so good at it), internet and common tasks such as browsing files.
Why CPU's nowadays are using more cores/threads is to deliver a better performance and yes for the most part more threads/cores = better CPU as while a Intel Pentium G4560 is great at gaming trying to multitask becomes harder and production is out of the question.
Use case is where things really matter and you cannot base everything in benchmarks as even if the iphone 7 works quicker than the samsung S8 what you can do with a S8 will be far greater if talking about production/multitasking.
Also keep in mind S8 is still new so optimization has not hit it yet while the iphone 7 has been out for a while.
There are factors where a octa core CPU will be better then a quad or dual core, especially in production/workload scenarios .
But yes there are times where it isnt as good as seen with AMD's Ryzen in its Ryzen 7 series, while I say a Ryzen 7 100 is a great processor for content creation (why I got mine for as I plan on making videos here in the future) it doesn't do as good at gaming as say a i7 and is more on par with a i5 (which I have no issues with) again it goes back to optimization.
However there is a good case for 6 and 8 core CPU's and more threads as high workloads call for such power under the hood.
One thing I like about ryzen 7 at least is that I can run my games without it stuttering or freezing while running stuff like chrome or office in the background as the processor isnt being overloaded by just playing the game.
Dividing the workload is a big advantage over dual core or even quad core and having more threads helps too.
Use case is something that has been debated of course if purely gaming quad core should be your minimum as games are starting to catch on to multi core/thread performance as consoles are starting to become more like PC's.
After all both the PS4 and the Xbox one feature a 8 core AMD processor and both do more than just game as both feature the ability to do live streaming and multitasking thus why there is a good case for if someone wants to consider a Ryzen 7 1700 and believe me i will be doing some live streaming and perhaps even using multiple monitors so i can see the chatrooms while I game.
Dual core however is indeed dying out, quad core is starting to take over its market and soon you may see quad core pentiums.