What if we had a player who was handed a document, skimmed the lore section and saw an awesome hook. They go to the DM with a cool idea to take advantage of this hook, using a specific race. (1) The DM then tells them no, that race was banned, didn't they read the document? The player admits they hadn't gotten to that part yet, but they thought this was a really cool idea, can they work something out?
I would hope they could figure something out, especially if the idea was really cool. But, a banned race, in my humble opinion, was put there for a reason by the DM. It is, after all, about everyone's enjoyment. But, hopefully, they could find a workaround.
The DM tells them absolutely not, trust them, that race would ruin everything. Player says that seems a bit like an exaggeration, and they'd not want to ruin the campaign, but this seems like a wasted opportunity.
We differ here. I have never heard a DM say it would ruin everything. What I hear is the internal logic of their world, the ability to suspend disbelief, the ability to DM without being annoyed somehow on a personal level, or the ability to let players overcome a challenge without a built in win button, deters the DM from allowing the race. I have seen a lot of DM's limit a lot of things. Sometimes I think it's nonsense. Others I can see it is a personal preference, and since the game is for
everyone, I respect it. Other times I see the DM has spent a lot more time and effort than most, and certainly anyone of the players that have "a cool idea" on developing their world.
At what point was the player being rude? Ask #1? #2? #3? Are they only rude if they wait a bit and then ask again later? A lot of people seem to have ironclad ideas about where the player is overstepping their limits, where are they?
I would say it is never rude to ask. But if the DM explained the reason. And gave you a short lecture. Then you question again, and they give you a longer historical version. The player should drop it. It doesn't matter of they don't agree with the argument. The DM has their world. It suits their logic. Here is an example:
In a world where creatures are created, not spawned. The DM decides to eliminate many of the more natural creatures from the MM: griffons, unicorns, etc. They also add spells to the magic user's lists that allow for said creations. If a player insists on arguing this, saying they had a lore-based image of their character wearing griffon feathers and stabbing with a unicorn horn, why is that appropriate. (I mean, it sounds cool to me. But, if the DM was clear that those creatures do not exist, only elementals, golems, etc.). I mean sure, the player can come up with ideas on how a wizard created the unicorn. It might even make more sense. But why insist? That is the question.