I'm sorry this is happening to you, but as harsh as it probably sounds: this is why you don't wait until now to have this conversation.

I run my 5e games very simulationist: I roll in the open and the dice tell us what happens. Our "story" is how we react to those happenings. I advise my players to keep an up-to-date backup character (for troupe play) or be leveling up several characters at the same time (for West Marches play) because while 5e is very generous with its dying condition, it's still a very real threat.

But

I make very certain to have this conversation before the player picks up their dice for the first time, ideally before they make a character, and in a perfect world before they even agree to play in the game to begin with. If that isn't to their taste, they don't have to play in that one. I run other systems that lead into other styles of play; maybe they'd prefer to play in my game where story is most important, the players are capable of winning almost any fight, and characters can only be killed via noble sacrifice. Which is good, because if people go into that game expecting 5e they're going to be disappointed. I also run a game system where no characters no belong to anybody and anyone can just declare a fact on their turn. That gets a third speech.

None of these are bad ways to play; if they were I wouldn't use them. But not of them are enjoyable to everyone. If someone likes one over the others that's okay, the same way someone might love Scrabble but hate Monopoly. It just means being sure to invite them on Scrabble nights but give them a pass on Monopoly nights.

Since you are here, my advice really is to stop the game (at least for the moment,) sit down, and talk about what kind of game the group wants to play. Nobody can "win" this fight, since they can't make you DM and you can't make them play.

The bad news is that the group may not actually be able to come to an agreement, or that the agreement may result in needing to change the game entirely. The good news is that your time isn't "wasted," you'll still have all the work you've put in so far to use in the future.

In the future, though, don't put yourself through this again. Have this conversation before you play. Good luck.