I can't see the benefit of going from 3960X -> 3970X, the 3990X though, if you really love your MT and you can get it for a good price, why not.
For newer stuff, if you look at the latest Intel Xeon W3400 benchmarks done by Puget here for Cinebench:
https://www.pugetsystems.com/labs/articles/intel-xeon-w-3400-content-creation-review
You can see the Zen 3 TR Pro 64 core 5995WX is first with 69826pts the new Xeon 56 core w9-3495X 2nd with 62392pts. But the 32 core 5975WX comes in 3rd at 48637pts. 24 core 13900K gets 29890pts and w7-3455 24 core gets 37045pts. From what I can see here (sorry puget didn't benchmark the 3000 series there)
https://www.cgdirector.com/cinebench-r23-scores-updated-results/ a 3960X gets 34932 pts and the 3970X gets 46874pts.
The 3990X is 75671 pts.
If we look at apps:
Blender
Da Vinci Resolve
You can see even modern apps don't scale so well especially just running one workload at a time. But Blender CPU render shows an ideal situation where all cores are saturated well.
To me this tells me switching from a 3960X to a 13900K or a 7950X for that matter for highly threaded multicore work is pointless. I briefly considered it because a single core boost is helpful, but can't justify for mostly downgrading MT. I can't see a justification going from Threadripper to Threadripper Pro at similiar core counts either. Cost difference is far too high. The new intel Xeons like the 24 core DDR5 w7-3455 cost £2700 without the £1k motherboards and that looks like a MT regression as well. So unless you need more PCIE lanes that Threadripper requires you're better off waiting.
3960X to 3970X I have considered as well, did not realise the 3970X (~£1200) dropped that much in price. Can't see the hassle of selling the 3960X for 8 more Zen 2 cores is worth it though. However, the 3990X is still a MT beast. Cheapest i've seen is £2300 from china. That is probably the best MT upgrade for TRX40 motherboard users.
I'm waiting to see what the Zen4 Threadripper Pro prices will look like, I'm sure they'll be obscene, but maybe Intel's new Xeon line will see a price drop. There is a small chance that a smaller 64 PCIE4/5 lane Zen4c Threadripper line will come out and be a bit cheaper but we will have to wait and see. But from my perspective HEDT is dead, and maybe for users who cannot quite afford a workstation for personal use will have to see what Zen5 will bring. Should be quite a boost from Zen2!