Post War and/or Post War type conventional for the long term

You make some good points. I too have a few conventional engines, both post war and modern, that I know I can rely on. I often have one or more conventional on the layout, but usually Williams.

But my view on DCS engines is a bit different. Even if MTH goes away completely there will be a good market for repairs and parts that I suspect some smart people would take on, many of which will likely be the current hobby shops that sell MTH. Engines can also be converted to other systems, even just conventional if necessary.

Not ideal and probably not always going to be cost effective, but I believe the market will fill the void in some ways such as these.

Vince, I really don't worry about this. It appears that there will be support for you DCS systems on the horizon. Or, these engines can easily be converted to TMCC.

The Pre-War/War equipment will be with us, but I much prefer the Modern Era stuff. 

Rest easy, the sun will come up in the morning again (if it is not cloudy).

You need to think beyond board replacement. Other parts will be needed too. Of course like some Lionel parts dealers others may take on reproduction parts for MTH. I still like the fact that a conventual engine that's sat on the shelf for years will come to life and go to work. An unnamed friend has a whole heap of MTH that has never been run, each one that is tested now, needs work. Well not all but most. Never used but he's paying now for upgrades. Young people today have many ways to stay entertained other than toy trains. My six grandsons don't even ask to see them run.

"Sad that the grandsons are not interested.

I guess back in Lionel's golden years, the fact that passenger railroads were all the rage to travel helped.

On another note, I wonder just how many hopefuls in their 20s can afford a $350-1000+ model train.

That's the nice thing about the smaller gauges, but you need young eyes to see them"

Look at some of the prices for the HO stuff. O-gauge is not far off.

When push comes to shove, I think the modern locomotives with can motors can always be modified with aftermarket packages as the OEM electronics gradually expire. Whether it's by cottage industry builders or larger concerns remains to be seen I guess - I'm not a big electronics fan or expert.
The older electro-mechanical reverse/wound field motors will probably be around for a long time yet, but might also see some re-manufacturing given sufficient demand. For example, the conversions we see of old American Flyer S gauge to can motors and DCC-type systems may spill over into postwar Lionel, who knows. Maybe RC / battery power will become the next big thing. Just my $0.02 or less - for myself I happily run conventional with cab control, so what do I know... 
FW

my thoughts are we seeing a repeat of the toy train market dwindling as it did when american flyer and others and lionel itself shrunk  late 1959 forward? if you think about it trains and new manufactures have slowly exited for various reasons so are we experiencing the same trend now?  and yet during the lean years we all managed to keep running trains we had and enjoyed them just as we are now.  I enjoy running not only the only modern day feature packed trains that at one time were only in our imaginary world but the conventional trains some of us grew up with. 
in 100 years from now will our once prized feature packed trains be replaced with virtual reality where our trains can have the largest curves available due to no space limitations? will the people of that era think our trains are obsolete we will never know as most all of us will be dust by then.

until my day of departing I'll enjoy what I have and run them until they are no longer able to with the current pc boards etc that made this hobby step up to the next level.