It doesn’t sound harsh, it sound ridiculous. I am not my players’ parent, and when one of them DMs, they are not mine. Any attempt to behave as if they are would be met with a hard stare and a firm, “No.”
Hell, I’m not a kid, either way. If my Dad tried to tell me what I was going to do, he’d get the same response. And he’d recognize that he was in the wrong, because he didn’t raise me to be the sort of person that accepts an external expectation of obedience.
Obedience is not a virtue.
Let's play this scenario out...
GM "I would like to play a campaign influenced by Game of Thrones. It will still have magic and monsters but the characters will be regular people in a medieval land.
P1 "Nice. I will play Sir Knight the Knightliest of Knights!"
P2 "Sure. I will play Lady Noble the Noblest of Nobles!"
P3 "Sweet. I will play Sir Sneak the Sneakiest of Sneaks!"
P4 "Okay. I will play Sir Elf the Elfiest of Elfs!"
GM "No. I just said it's a human-only campaign"
P4 "Well, if I'm going to play, I want an exception. I'm an elf. So I get darkvision, get +2 to perception checks, have advantage on Saving Throws against being Charmed, and sleep won't work on me because I don't need sleep."
GM "Fine. I want to keep the group together so you can be an elf."
P2 "Hey, if he gets an exception, I want one too. She's a noble and likes hunting. So I want her to have a rifle. And she's going to be a variant human and I'll take Gunner as a feat."
P1: "My knight's backstory is that his family was killed by orcs. So put in some orc encounters for him."
P3: "Can we start out at level 5? I want my character to multiclass as a shadow monk/rogue so I can have a Batman build."