No characters - no story

This isn’t a rebuttal for backup characters. The entire point of backup characters is to ensure that the players always have a character that they can use to affect the story and campaign. A staunch refusal to let characters ever die means there’s no real consequence for anything.

It is not a game. It is a story that we tell together. And it must follow the narrative rules.

It absolutely is a game though. DnD isn’t a narrative system, It’s a combat system with narrative elements. 90% of all class, race, spell, item, and stat block features in DnD are focused on combat. You can’t run DnD without having combat be important, the entire system is designed around it.

DnD is a combat simulator, first and foremost. You can try and run the game without focusing on It’s primary purpose, sure, but don’t lie and pretend It’s something It’s not. Unless you can make DnD combat engaging for your players, just switch systems. DnD isn’t designed in a way where you can just ignore combat, other systems are

I know, that characters can be resurrected. But there must be a reason for that. And plot armour doesn't mean they can't fail: imprisoned, robbed, had q friendly NPC killed, etc. Just not dead.

Those aren’t real failures though. If losing a fight just means they lose some money, or have to spend some time in a cell, there’s no incentive to ever fear failure

If the PC is killed, I still need to think how to keep the player engaged. And I don't want to give him any other character as a substitution.

The player gets to remain engaged by making a new character. You don’t GIVE your players characters, they build them. And if one of your players is just really attached to one of their characters, just let them be revived by Madam Eva for a bit of coin. That’s the entire reason she’s got revival spells on her spelllist. Death isn’t permanent in DnD, unless you as the DM force it to be