If they’re not getting the ‘performance you’d expect’, then your expectations are wrong 
The i9’s (and goes for AMD’s Ryzen 9 as well) really are productivity CPU’s (10 cores / 20 threads and even more). Games in general (sims included) can’t utilize all those extra cores and threads. What does matter though is single core performance (to help with the main thread). The 10900K does pretty good at that, but the CPU will show low utilization due to its additional cores.
It’s also why the Ryzen 5000 series CPU’s work so well for the sim; they have great single core performance. But for AMD goes the same as for intel; anything over 6 or 8 cores (12 or 16 threads) is overkill for any game or sim workload (at least for the foreseeable future).
Regarding the 7700; it’s a 4 core / 8 thread CPU, which is 4 years (3 generations, almost 4) behind. It will do pretty good, but don’t expect wonders.
If you’re considering a new build; take a look at the Ryzen 5 5600X (if you can get your hands on one), it’s pretty much the performance sweet spot right now.