There are indeed fields of computing where Intel's X299 lineup still performs better than AMD's Ryzen 3000 series. And even offers better performance per dollar. Engineering/scientific simulations like CFD and FEA is one of these areas. Source: I have been working in this field for many years now.
You already mentioned the key specification here: memory bandwidth. AMDs Ryzen CPUs stop scaling at around 6-8 cores due to a lack of memory bandwidth when running parallel CFD codes. See for example the results here: https://www.cfd-online.com/Forums/hardware/198378-openfoam-benchmarks-various-hardware-14.html#post763993
Intel CPUs can draw a lot more power than their rated TDP for short periods of time. That's how turbo boost works. So can AMD CPUs. Unless you are overclocking heavily, you won't see 300W sustained power draw from an I9-10900X. Not even close https://www.anandtech.com/show/15039/the-intel-core-i9-10980xe-review/2
Less than 50W idle power consumption for a whole system on an HEDT platform might be a bit optimistic. You will probably end up in the 70W range with a typical configuration. But this figure does not depend on the CPU alone. Motherboard, drives, GPU... all play a role here.
Why doesn't AMD release a Threadripper CPU with less than 24 cores, at a more reasonable price?
I can only speculate, and there will be a lot more reasons than I can come up with in a short paragraph. But one of the most important ones will be this: AMD is a relatively small company compared to Intel. So far, their goal is not to cover every niche possible with the perfect product. That was Intels goal. And let's face it: scientific and engineering computations on moderately priced workstation PCs is a niche application.
More L3 cache is nice to have, but outside of edge-cases, it can not completely make up for a lack of memory bandwidth.
Btw: if you are looking for a cheaper TR3000 alternative that better fits your requirements, AMD still has something in their lineup: Epyc 7302P. The CPU and the boards are not exactly cheap, but still much better value and higher total performance compared to the cheapest possible TR3000 build.
forgot to mention this: especially with your applications in mind, I would highly recommend faster memory than DDR4-2933 or DDR4-3200 for both AMD Ryzen 3000 and Intel Cascade Lake-X CPUs. DDR4-3600 is around the sweetspot for price/performance.