As a relative upstart in S scale, here's some input from the peanut gallery:
IMHO, any interest that is based on aging Baby Boomers is being eyed by the mfg'ers catering to that interest. Like it or not, the numbers of people that like nostalgic models from their youth are aging and dying off. This will/does mean less market for newly mfg'd nostalgic type pieces. The mfg'ers have maybe another 10-20 years (or less?) and the previously strong nostalgic market will definitely be a niche market, such as tinplate today. (VERY few tinplate enthusiasts compared to the overall hobby of model railroading.)
Whether any of us like it or not, the long term future for model railroading is more scale fidelity and newer prototypes. I feel this holds true for S as well. IF a mfg'er is going to make inroads into the fertile fields mentioned above (i.e. capitalizing on its perfect size), it will do so by attracting the non-S scale serious scale modeler and not transplanting the nostalgic enthusiast. (Such as an O 3 rail guy into AF type S.)
What will likely be a dependable era interest (regardless of scale) for many years to come will be the steam/diesel era. However, it too, will shrink as those adherents age/etc. Thus, the best future market will be newer prototype models along with extracting the remaining sales found from the transition era guys.
Of course, all of the above is merely my opinion... so... I could be wrong!!