I think you really missed the point on that.
"I get that you are trying to be balanced, and I appreciate that you accurately described the other side, but my side is the one that is totally reasonable! So I accept your classification of one, but we have a lot of nuance!"
Look, I get it. But you did see that the other people on the "curated side" repeatedly said that they usually work with players, too, right?
A: I like peanut butter only, but will add jelly on occasion.
B: I like jelly only, but will add peanut butter on occasion.
A: ..... Well, I guess we have to go to war, then, you dirty jelly-lover.
This is a truly stupid argument, because it's not really about what it's about, as I stated. It's about player empowerment, and DM "rulings not rules," and different playstyles, and trust.
But to answer your last issue- yes, I happen to firmly believe that an entire sea of water can't sink a ship unless it gets inside a ship. I've gamed for far too long to have a bad player at my table (or to be a bad player). IMO and IME, if a DM and the table agree on a theme or "curated" setting, then the players create PCs that match the agreed-upon rules. That's how things work. Other people are welcome to play differently. To the extent that you might view that as my belief that the "player can play something else," then I cop to it.
EDIT- OTOH, if a player comes up with an idea for something that works within the setting but was not otherwise approved, then I will work with the player on it.
If someone came to me and asked if they could play a tiefling I'd point them back to the allowed races list and have a discussion about why they want to play a tiefling. Want to have demonic blood because of the role playing opportunities? Okay. You're a cambion, but for some reason it has no mechanical effect. Why that's the case will eventually be revealed, there may or may not be some options that open up at some later date. Pick a race from my list. In the meantime let's talk about some minor physical attribute such as your eyes glowing red when you get angry.
Want to be a Kryptonian? Okay. You're from the tribe/city/island/nation of Krypton that was wiped out. As far as you know you're the last one*. What do you remember? Let's see how can I work it into my story. It might take some form of suspended animation/time jump but I'm sure we can work something out.
*Except of course for your cousin, a dog, a few dozen other survivors led by that Zod guy and depending on story line an entire city. But other than that, definitely the last one.