N3V is looking into a way to bulk replace missing and faulty assets we were told a while ago. When this occurs, who knows.
The reason for the stricter error checking was due to crashes. TRS2004 had minimal error checking and would horse through content errors. This not only caused performance issues due to the horsing through and checking, but also caused instability. TRS2004 would load up fine, but crash randomly to the desktop, or usually on exit with the infamous GPF errors. The long-term workaround for that error was to do a Save-As rather than a Save before exiting. Then due to many user complaints, error checking started in TRS2006 and got more and more strict with intervening versions. Today, content-related crashes are pretty rare compared to the olden days. To allow for backwards compatibility, older assets are allowed to co-exist with newer ones in the newer versions as long as they follow the rules for the build that the assets were created at. This works for 90-99% of the assets.
One of the problems was that early Auran content creators made errors in their own content and many, many content creators cloned those assets, causing more faulty content than was necessary. If Auran had been strict about errors back in the old days, we wouldn't be in the mess we have today.
The issue with the older Speed Trees is a different issue altogether. When N3V released, T:ANE, a 64-bit version of Trainz, they had to upgrade their 32-bit version SpeedTrees, the software developer IDV had no direct upgrade and any tree developed using the 32-bit libraries is now faulty. Sadly, IDV is notorious for doing this even between interim versions. Trees created in version 6.5 are not compatible with version 7, or whatever version is out there today. This makes creating new trees difficult for us because N3V is locked into 6.5 or whatever version they use, and any tree created above that version won't work at all.