A great point is why don't people change the look over their old models to make them look better? I mean you can change the ENTIRE look of anything through an update to it. You could make the UP Big Boy into a Shay through an update if you really wanted to.
If you REALLY wanted to, maybe. But it would be a lot of work, and I mean a LOT of work to take the old Trainz UP Big Boy and modify it into a Shay. The difference between the two are monumental and by the end, most likely there wouldn't be so much of the 4-8-8-4 left on the Shay, in fact, it would be far easier, and much less time consuming to just build the Shay from scratch.
The same would apply to the current Big Boy, the amount of changes that would need to be made to the existing model to bring it to current visual expectation standards, it would be a lot easier to just make a new one from scratch. Of course, there is nothing truly "easy" as far as content creation goes, but there is "practical".
The current one isn't "bad", it's just no longer up to today's standards. Maybe someday a new one will be made, but until then, we can simply enjoy what we have.
Now, for why people don't "upgrade" their old content, there are many, many reasons for that, many are justified. Some content creators simply do not have the time to go back and completely redo something they spent literally thousands (or even more) hours painstakingly making in the little spare time they had. Some have decided they have better things to do then spend their time making free content for an often thankless community of freeloaders, who have abused said content and sometimes even attack the author of said content, because they do not understand that content creation is not done overnight, is a time consuming process, and it seems, people are just lacking in respect. There are many other reasons but too many to list here.
I consider content creation is a hobby, something that is supposed to be enjoyable and fun. No one is obligated to do it, and if it becomes way too stressful and more like "work" than a "hobby", it's might be time to take a break.
I just wish minimum standards could be better in some instances. Especially with older content like the UP 4-8-8-4. The thing looks like paper.
I think many of us "wish" such things, but you should keep in mind, the Trainz franchise is about 16 years old. It's older than I am. And until T:ANE it used a game engine that IIRC dates back as far as the '90s. Back then, in the early days of Trainz, your average desktop PC was less powerful than the $20 Android smartphone I am using to type this post (no joke).
Trainz back then used the latest graphics and software capabilities at that time, but still it was very limited compared to what we have today. Auran (now N3V) have upgraded the software many times and have implemented new features and scripting capabilities, as software technology advanced they took advantage of it.
T:ANE uses some rewritten components from the old Jet engine but it's to allow for backwards compatibility, and so the Trainzers who have been using it for a longer time do not have to completely relearn everything. Since it does inherit features from older versions of the game, naturally as one can expect, it will inherit the limitations. Again, any features that were not implemented early on to be set as a minimal requirement, it would likely cause too many problems to encode into the game now.
The "Next Trainz" currently referred to as TRS2018, will to my knowledge be backwards compatible also, maybe that can include the feature you're talking about, although considering the issues that will almost undoubtedly arise from it, the fact that it already can be done by scripting, and the time and money it would take to implement it, honestly, while I don't want to "rain on the parade", I can't really see the developers at N3V going through that kind of effort to add something to Trainz that technically already is available.
Although I would not be unhappy to be proven wrong