Handbrake will continue to use plenty of cores (and it will likely love Zen2's 256 AVX execution units as well). But handbrake doesn't have much to do with gaming. Gaming will continue to work fine with 6 cores (probably 4) without issue.
If you want 144Hz, you will need plenty of clockspeed (I'd expect AMD to work fine, but you'd be better off with the i5 he chose), so a few fast cores is a great choice.
Handbake and other video editing/streaming software have everything to do with gaming. Without these softwares a large part of gaming culture is gone.
That's not how it works. Sure, having an 8C/16T CPU will be great for those that chose to video editing/streaming, but a vast majority DON'T do that. Most people don't need an R7/i7 to do their daily tasks, including gaming.
It doesn't matter what we think. Amd is pushing 8 core as standard in 2019 and 6 core low end. The industry is now in a core war. Two to four years from now 6 core will be out of production and 16 cores will be standard.
After 15 years stuck with 2 and 4 cores as standard because Intel didn't have competition. There finally excitement in the cpu industry. AMD is back and better pushing the cpu envelope. 2 years ago amd made 6 core standard and Intel trying to catch up. In 2019 8 core standard. We are getting back to the fast pace cpu turn around.
I understand that more cores are becoming the norm, but it takes time for the existing user base to switch over to that. It was only last generation that Intel finally made 6 cores on their mid-tier processors, and not everyone upgrades every 1-2 years. So, in that time frame, programs/gaming will begin to start leveraging the additional cores as needed, but what we're all saying is that as of right now, more cores isn't necessary. Following what you said, I would expect at least 3-5 years before it becomes mainstream.
And, in that time, a lot companies will explore the possibility of using the additional resources, but it won't be an immediate thing since it doesn't impact a large enough audience for them to dedicate their own time and money.