Sergio dying would absolutely not be the perfect ending. Not because we all love the character so much, but because it would mean the resistance lost. The war he started first for his father,then for the love of his life would be lost and meaningless. The loss of his brother too.

He IS the resistance. He started all of this. To have the same ending as his father maybe sounds like a full circle,but it would all be for nothing then. Every single death.

Does it sounds unrealistic for them to succeed for the second time? Absolutely.
But to kill him in the end would mean to kill the hope that there is a way to beat the system. A corrupt one. The system wins then? But that would mean a tragic end for all of them,don’t you think?