From an objective viewpoint Jack is definitely not wrong to be willing to sacrifice one man to save the world. That’s actually pretty basic morality. And even if Dean did die, he would be back ten minutes later with Starbucks.
But Cas and Sam would probably re-enact Romeo and Juilet and fecking drink poison because they thought Dean was ‘dead, dead’. And then Dean returns to the bunker, sipping his frappuccino, sees Sam and Cas dead on the ground. Kills himself out of despair just as Sam and Cas also come back from death.
When Sam and Cas awaken they see Dean’s body, and kill themselves. Dean returns, sees Sam and Castiel’s bodies etc.
Rounding in an endless cycle of pointless deaths, while Jack stands in the corner traumatised.
And that Jack is why we don’t kill Dean. Look, ya made Cas faint at the thought.