Bananas are a black hole that takes the files but does not allow them to be removed. Why do you prevent authors from deleting bad old files?
What would you think if <random person> would come and tell you "no, you cannot continue to play your favourite savegame"? That's what you're advocating here:
It's smple: people will have maps with that require a particular version of a NewGRF. We want to make sure that people can continue playing their games they started. So any person who has a game which requires it, will be offered the NewGRFs needed - whether it is considered obsolete, broken for whatever reason or not - as otherwise the map itself is broken. To ensure that people can continue to play, we will never remove a NewGRF or AI or GS.
Authors can disable general availability of their NewGRFs, AIs or GS for all other cases as you can set the minimum and maximum OpenTTD version your content is offered to.