I look at it this way. For those of us who have used the Trainz program for close to twenty years or since the very beginning, we always could merge built-in and supplied routes. It was this feature that set Trainz apart from other programs. Heck, we could modify anything and everything. Many of these assets became the basis for many of our creations in the early days, or later on with more sophisticated routes we took parts that we liked from supplied routes and combined them. The old Tidewater Point Railroad (the original one) came in two parts for example, because it was "two big" at the time to fit on a single CD with everything else. For many of us, we felt this was better as a single route and merged the two together. With a bit of blending and other modifications, we then created an empire out of these two routes.
Then with TS12 SP1 HF4, things changed. N3V introduced the DLC packages, and it was nope can't do that anymore. There were other factors that brought this on including outright piracy and for that I understand why it was done, but this was an overnight change without warning which is typical of N3V management, so this shouldn't have been expected, but there was no thought put into this either, typical of N3V management. This then became a bigger issue with TANE and everything getting locked down including built-in assets, making route building difficult due to fear of including something that was built-in, and also because there was no way of telling whether an asset was built-in or DLC, or not.
At the time, when this Sh*** started hitting the fan, I came up with a suggestion which was duly ignored as usual. I saw crickets, probably because it was a solution, but it meant work on something that was considered finished, as usual by N3V management. Why not include some kind of key associated with those routes when installed? This could be a hidden file, or something similar that can't be removed. If it's removed, then the route can't be uploaded. The route can be cloned, which will include that file, the route can be merged in, but that also will contain that same file, and we can have our ability to merge and play around. In addition to the key-file, if we want to call it that, the current copy and paste can be blocked so that DLC and built-in routes that are blocked can't be copied and pasted baseboard by baseboard.