One has to wonder if there's really a gap at all.
If there was truly a gap in supply, wouldn't someone have purchased MTH's 3-rail inventory, tooling, etc.? After running only Lionel and WBB conventional and TMCC for 15 years, I made the decision to add DCS to my in-progress layout 2-3 years ago. I love the Rail King Imperial offerings, have already bought 6 engines, and was planning to slowly replace my old TMCC with PS3 Rail Kings. Nice timing.
Now, I have to wonder if there simply weren't enough of us consumers to keep MTH in business making engines and rolling stock.
I've seen Industrial Rail, K-Line, and Weaver exit 3-rail since I joined the hobby in 2002. One could argue Williams is effectively gone as WBB's offerings pale in comparison to Williams from 10-15 years ago. Sure, MTH was probably bigger than all those combined, so that's the biggest drop in supply our hobby has seen, but maybe that's just supply further contracting to match contracting demand?
I want to state I'm sure dealers here would know better than me, I'm just speculating based on my own habits.
With all the product on the secondary market and, sadly, an aging group of hobbyists, I think there will be a glut of basic, good-enough chassis on the market for a long-time. Lots of PW, MPC, TMCC 1.0, and PS1 and PS2 engines are available that can serve most of us for the rest of our lives provided enough "guts" and spare parts are available to upgrade. I think entry-level sets and high-end, $2,000 engines with new gadgets and features will be the wave of the future. Here's why:
I have an early Lionel TMCC semi-scale Pennsy K4 with TMCC 1.0 just like the one in the 2021 catalog. I could upgrade my current one for about $250 to get most of what a new LC+2 would get me. Just yesterday, I bought a Williams Berk from a fellow forum member that he upgraded with TAS EOB and turbo smoke for half the MSRP of a new Lionel LC+ 2.0 Berk. My gain from him abandoning semi-scale for full scale.
Aging hobby, many with all they need, and competition from the used market.....all factors that may be leading to a major contraction in supply of new product.