Right but enabling / disabling HT is only a few clicks away in bios, so what do you have to lose?
If you play, and only play, Arma 3 or Dayz, for example, which many people do, you would be better off with HT disabled and a faster OC. This can specifically help not only FPS but the frame latency.
"but there isn't a magic value you can arbitrarily assign to games or "most" games either. Clearly we want more than two cores for gaming now, "
You speak as if the 9900k is a dual core cpu with HT disabled. Did you forget about the other 6 cores?
BF1 is THE GAME that supposedly, the more cores or threads the better, but the 9900k actually performs worse than the 9700k, despite having 8 more threads and more cache. Not only that but the frame latency is almost half; 4.7ms vs 9ms in some cases. https://www.youtube.com/watch?v=568uEmI-et4
GTA 5, 9700k 108fps 9900k 115fps, however 9700k always 1 second of latency less at the same fps. https://youtu.be/568uEmI-et4?t=85
The difficult thing to know is whether the extremely slight FPS increases in favor of the 9900k are due to the extra threads or the extra cache. My guess would be the cache because HT introduces latency which you can see in the video on each game.
4.7ms vs 9ms is a BIG difference on a 1ms gaming display.
As far as your comment on "Most games use 2 cores maximum" kinda falls apart. Years ago with games like GTA V and other triple A game titles from that era has been proven to fully use 4 CPU cores. They were often proven by showing FPS differences between the i3/i5/i7 of the time and it could peg a 4 core unlocked overclocked i5 to 100% on all cores. These days games like Assassin's Creed Odyssey on benchmark videos where it can slam a 6 core i5-9400 at 100% use all cores. Can't say that is all background tasks but games now are using more and more cores as game developers know people are likely to have them.
Before 2010 the logic was that most games could not use more than 2 cores but times have changed.
Now the differences from the current 6c i5 and 8 core i9 are not huge and an argument can be made about cost to performance whether the i9 is even worth it as you can still have a great gaming experience on a mid range 6 core CPU from either Intel or AMD.
As for disabling HT for added performance it really depends on the tasks that you are doing. Your examples do seem rather far off too. 111 FPS is on average 1 frame every 9ms and 212 FPS is a frame every about 4.7 MS. Sure displays having 1MS response times are nice to reduce ghosting but saying disabling HT does not double your FPS. If your PC is pumping out 144fps then it is delivering a frame about every 6.9ms and that 1ms response time on the display allows most of that frame to be fully visible with limited ghosting from the last frame. Well it is true you might be able to see the difference of 111fps compared to 144+fps on a 144hz display with a 1ms response time but not on a 60hz display with the same response time.
I went from a 5ms panel to a 1ms panel and yes I am aware that the MS rating is cherry picked. Though the cherry picked 5ms is definitely slower than the cherry picked 1ms. This is a good website to see ghosting and it makes the difference between the screens easier to see. I do see the difference in fast paced games as well so it does have some value. I do agree though to take it with a grain of salt.