After everything they went through, I think it would have just been too dark if one of them had died in the end and left the other alone

That Joss cliche worked more with couples like Willow/Tara or Wesley/Fred because they did have the more sweet and fluffy relationships. It was an effective slap in the face for the viewer when Tara died the way she did when you're awwing over the Willow/Tara, or when Fred spits up blood right when she's singing and happy, but with Victor/Sierra it would almost have been expected as just another piee of crap for them to deal with considering that their relationship was the one bright spot in the hell they were both living in/trying to forget (especially Sierra ).

If that makes sense