There have been many a route and asset(s) I have downloaded over the past 15 years of Trainzing. Some stuff remained while other stuff just didn't cut it.
Will TRS2019 (TRS19) kill older versions?
I doubt it. New users are where we were when we purchased TRS2004, TS12, or any other Trainz version. These people are just starting at a different point in the game's progress, and this is not unlike the progress of many programs, although Trainz is unique as a franchise. Unlike other programs, however, we can still run the older versions of a program if we want. It may take some tweaking, but it can be done. What's stacked against us in that respect is everything else around that older version has moved forward, and our methods and resources are different, which makes running those versions more difficult. Take TRS2004 for example. It was great when it first came out in December 2003. It was the bees' knees as they say with interactive industries and the new driver command bar which finally had the ability to be saved in a driver session, but the very operating system we ran it on has been obsoleted. Yes we can install Windows XP, Windows 98, and even Windows 3.1 if we want, but that comes at a cost in other areas. The computer hardware is faster and has more memory. These older operating systems don't like this and suffer from ills which are a bit complex to explain here. The alternative is a virtual machine running on Windows 10, for example. Sure these are great, but there are limitations there as well including video memory, and here we are going along this path of no return just because we want to run something older. I liken this to the old lady that ate the spider with one thing used to fix another, but to no return and with too much work.
With that said, TRS19 is more or less up with the current times. Being a 64-bit program, it makes use of all the resources you can throw at it. It also uses the graphics cards we have today in ways never imagined for Trainz. We're no longer limited to 2500 km and we have shadows, grass, and other stuff which was impossible in other versions. This was something the community asked for when T:ANE was being developed. "Give us something that isn't TS12 with a new interface" was said over and over. We felt that even, at the time, TS12 was old, and we were right. TS12 had its day in the sunshine, it still works but with more and more fussing to get going with file permissions, and video drivers. Eventually the download for DX09c will be difficult to find. TS12 got bogged down just as any computer program does as things get bolted on and stuff gets fixed. As the OP should know, and many of us do know, fixing a program is no easy task. Adding in something or fixing something, can and will break something else down the road just like fixing that old Chevy Malibu station wagon that's held together with Bondo.
There's nothing stopping people from running the old stuff, but don't expect it to be updated. Companies need to move on and stay forward looking, and N3V is not any different. The company is small and can't afford to support older products as they used to. It was great when something running Trainz 1.0 could ask the help desk for help, but that's no longer the case. It's no different than asking Adobe tech support for help on Photo Shop 3.0. It costs money to support these older versions whether the program is used or not, and with declining usage of that version, it makes sense to put the efforts into newer versions.