EzekielRaiden
Follower of the Way
That modifying the rules is mentioned in the books is rather a non-sequitur for what people can reasonably expect to be on offer unless told otherwise.I can see where you're both coming from. I don't agree, though, because anyone truly new to the game isn't going to have hard expectations of it, and anyone experienced with D&D has assuredly run into, well…
You have, literally always, been allowed to simply not serve rice when cooking teriyaki for friends and family. No cooking police are going to swoop in and tell you you have to eat rice. No cookbook would ever dream of suggesting that you're not allowed to not offer rice. Yet despite all of that, people will expect rice unless you tell them you aren't serving any. There would not be a single error or even wrinkle in judgment for them expecting rice, even though "chicken teriyaki" doesn't actually say "rice" anywhere in the name, nor is rice technically part of the dish itself, but rather the staple side that goes with it (and, in general, nearly all Japanese cooking).
People keep conflating "the books say the rules can, and even should, be modified as needed" with "having expectations of what D&D is or contains is crazy talk," and that's just not valid. The two are different things.
This is why I kept harping on the published books--very specifically the PHB--being the "common starting point." The rules that are specifically and explicitly meant for presentation to the players. That's square one. It is not irrational, inappropriate, rude, excessive, demanding, or any other pejorative adjective you might like to volunteer, to have "the stuff in the core player rulebook is there for players to pick" as a built-in assumption.
But D&D, unlike most other games, is a journey without a singular, fixed destination. You can choose where you want to walk. And you can choose to pick a different starting point. You, as DM, are encouraged to do so if you can make it work for you and your group. But you're still needing to re-center things. The PHB options are, to use a mathematical analogy, the origin on a Cartesian plot: the natural and presumed starting point. Just because it is natural and presumed, though, does not mean it is required. Many things--even many vitally important things--never touch it (consider y=eˣ). But just because it isn't required in some absolute sense, doesn't mean people won't look for it, nor experience (often momentary) confusion if you fail to include it in a plot. Likewise, especially with options as famous and storied as (Tolkien-esque) elves, you should assume that your players will expect their presence unless informed both reasonably comprehensively and reasonably early.