Yet another thoughtful and thought provoking post.
I am a fan of Stout and have been watching a bidding on their "trains" for the past 3 years or so when I got back into the hobby. Take a look at the hammer prices on the post war trains - July 14th auction. Remember to add 19% and then $30 in shipping. Not sure I would agree people are losing interest in semi-scale. Appears to be pretty strong prices for trains that are pushing 70 years old.
If the question is more toward Lionel's new semi-scale (traditional scale) and scale offerings - that may be an entirely different question. All I have done with Lionel is looked at their new catalogs but never bought new. Admittedly some of their steamers are simply incredible and would be fun to own.
Over the past year and a half I have transitioned from semi-scale to scale 3 rail, selling off much of my used semi-scale Lionel and MTH pieces. I do have a few pieces left which are destined for under the Christmas Tree layouts. I am unlikely to buy any new or used semi-scale (unless by accident). For that matter my scale fleet is nearly complete, so that buying is also tapering off. With 3 MTH engines on order those may very well be the last new engines to be added.
Just an observation regarding MTH equipment, it gets to be an interesting purchase decision when some of the used PS1 and PS2 premier engines are being sold for "close to new" pricing when replacing electronics are factored in. Given the newer engines typically have far more detail than the earlier types - why buy used? Of course one may have to wait for a while for a PS3 version to be offered.