Just like 5GHz with crap IPC was irrelevant 8c will likely lose relevance very quickly too with the current core arms race. As the core counts rapidly increase over the next couple of years, programmers are actually going to use that power, in turn 8c will die out pretty quickly especially with the better core densities of Zen3 and Zen4 in the coming years.
Just looking at the bigger picture rather than focusing on 2019
It may be silly, but considering the gaming side of the situation: as many still playing at full HD or 2k, the adoption to 4k by PC gamers is slow, as many desirable resources are often limited or restricted to really expensive monitors. The TVs are cheaper, but the main appeal for most is the price and the bigger screens, but content still very limited.
Assuming people gradually speed up the migration to 4k or even 8k... now back to the CPU situation, in 4k, all setting to max, a good 6c or 8c will bottleneck the top GPU available? Not for a while, as I don't believe any GPU will handle 4k full blast at 144, any time soon, and midrange GPUs, even longer.
At the moment, even the 2080ti can't do that much on 4k, and let's not forget the push to 8k displays.
To be honest, it's very complicated and almost impossible to tell for sure, but for the user of basic systems, will be ages before 6 or 8 cores to be standard. Just think about how long is taking for the SSD adoption, and the performance gain is brutal. But going 8 or 16 cores for web browsing and basic software usage, I can't see the gain.
As I'm aware, always will be people around happy to build rigs able to run games like Crysis or Metro, during their release, on ultra, at absurd resolutions, but those less than 1% won't dictate what any other developer will expect from minimum or recommended requirements.
Always will be those benchmarks disguised as games, but generally, consoles will hold back.
In general use, since Windows Vista, the OS doesn't put that much strain on the CPU. The bottleneck in 9 of 10 modern pcs is the HDD, and the rest possibly memory.
But for those who really need or want that extra power, always will be or the very top end, or server hardware as always has been.