This happens a lot more than you would think, and the problem is there comes a point where keeping stock of parts is more expensive than it is worth (and I am not defending Lionel or condemning them). The problem is when you are talking engines 10 years or older (which ironically are obviously more likely to fail) , they have to keep boards around and if they run out, may not even be able to get them made again in any kind of a cost basis. The boards have some standard components I would assume, but there are also custom chips on there, ASICS and the like (not an engineer, and GRJ is likely gritting his teeth; that's okay, my dad wherever he is will be as well. I do know that even if the same company that made the boards in the first place could make more, they would charge an arm and a leg (and a new company that might have to re-engineer the board to replace standard components that no longer exist, would be really expensive).
What we have here is what you see with appliances and the like, manufacturers assume that when the units fail, that people will buy a new one (and here I am talking things like the control boards they use in washers and everything else). They do something similar to Lionel, in that when they produce new models the boards are not the same as the older versions. So they keep boards for a particular model x years past the new model introduction, to fulfill warranty claims, then don't have them any more. Lionel prob figures that an engine that is 10 years old (or older), that if the person can't get new boards, they will buy a newer version of a similar prototype. Like the appliance companies, they don't think "Gee, these are expensive units, and maybe they should last a lifetime", they think "Okay, I made my money on the model, I kept the parts around for X years, my job is done".
BTW it isn't just electronics, I have a whirlpool tub that the faucet base may be shot. I talked to the company that made it, and they were like "Oh we stopped having parts for that 15 years ago". They assumed you will just go out and replace the whole tub (not even talking electronics, talking basic plumbing). Tubs like this are a big item, and people keep them a long while..and they don't make replacing that base easy, either.
The reason people mention DCC is easy, because it is a standard you have multiple vendors out there and because it is a standard you can likely get replacement/upgrade boards (I also understand that standardization comes with a *, because there are extensions in some brands that other brands won't support, like wrinkles on sound and so forth). In terms of power, there are DCC units that handle 2 rail O, with modern can engines the power consumption shouldn't be a problem (I doubt anyone makes a DCC board than can handle a pullmore motor). Most engines made in the last 20 years will likely be can motors though. Even if you lose some features, you can still have things like speed control and sound with DCC if you have to switch to another vendor's decoder/boards.