For one thing, there is no "plastic" steam to speak of, other than the Atlas 0-6-0. That has a reputation for being a horrible runner.
As for Diecast, so far most of it has many compromises that do appeal to more prototypical modelers.
The brass from the 60s on is as detailed as the newest 3 rail diecast in my opinion. That is US Hobbies, Sunset, NJCB, Westside, etc. Precision Scale also has done a lot of brass. The details are all lost wax castings and added on as on prototypes, not cast into the boiler. The thickness of things like running boards, window frames, tender sideboards etc are more prototypical.
And finally the brass has traditionally been done in short runs of maybe 100-200 of very specific models. They don't take one model paint if 47 different ways. The models are generally based on specific prototypes for specific RRs. The short runs were/are possible because the units are fabricated by hand with jigs and fixtures. there is very little tooling cost as opposed to plastic and diecast.
So if you model in in 2 rail and want good looking steam, brass is a good way to go.
However, as an old timer, in my opinion brass has also come down in price. Pieces that sold for 7-800 5 years ago are coming down to 600 in some cases. Many of the Older US HObbies pieces were getting close to 1000 some years ago and have dropped considerably since. I recently bought a "new" (never been run) USH Hobbies PRR 2-10-0 for 475. The Sunset PRR 2-10-2 that came in MSRP around 1000 somewhat more recently regularly appears for 7-800 too.
The Kohs stuff is still listed at very high prices and as you say is the Lionel Smithsonian Dryfuss. Those prices do seem high. But as mentioned above, if only 100 were produced, they are somewhat rare. Also, in the case of Kohs models and probably the Dreyfuss, the detail is museum quality. they are aimed more for display than operation.