But changing NewGRFs is like messing about in a time machine. It's doubtful that it can ever end well...
Yet you recommend it to arbitrary players as a valid solution. Note that they do not have the insights that you have.
To them it's a simple trick you can do to get what you want, without even realizing what they are doing.
It's like people playing with grenades that don't know it are things that may go boom.
The problem here is mostly that it often goes alright (or seems to go alright). People expcet things to work or break immediately. If it doesn't break immediately, "it's alright", and they forget all about it.
However this stuff MIGHT break in the future, months to years after they did it.
At that point they won't even remember that they messed it up themselves, or who told them or how they found that knowledge, or whether there was a red warning window three months ago. They have a game that they sank dozens to hundreds of hours into, and it's broken!!!
Do you really want to send players into that situation?
I know it's tempting to help, but I believe it's better to say "just start a new game". Yes it's not nice, but it's far better than ending with an irrepairable broken game dozens of hours later.
Worst that happens is that they have to play more openttd, but things do keep working.
And indeed, we would like to fix the scenarios very much. In fact, there exists a plan for it, and it's on my list of things I work on. Unfortunately, I have many things I work on, and it's not a simple addition, lots of internals need work before you fold the new functionality nicely into the system.