The imperial succession included the male linage only. Thus Nicholas' heir was Alexis. The four daughters could not inherit. Thus could not the four Grand Duchesses have been spared?
That would have made absolutely no difference.
From an ice cold perspective the children were destined to die because:
A) The ruling family should be exterminated. There was no room for them anyway in the Communist Russia.
2) Female empresses was far from anything new in Russia.
3) The daughters could deliver a male heir at some point.
4) The daughters were living political symbols for the anti-communists to rally around and fight for.
5) One of the daughters could marry "a strong man", who in his own right could be a dangerous threat. His claim to power would now be legitimized by his and the Imperial bloodline being mixed.
Okay, could they have left the daughters alive? Were there alternatives?
A) Prison, indefinitely. - No, the daughters would still be living symbols, who should be freed.
B) Making the daughters "renounce the world", take a wow and enter an cloister. - The Communists didn't trust and didn't want the church around. It was too big a competitor. And the whole thing about religion goes against Communism as a concept. - And they could eventually leave the cloister. Wows can be overruled if it's "Gods will" and it would be...
C) Exile the girls abroad. - See A, except that you no longer have control over them.
D) Letting them disappear somewhere. - They could still be recognized and there would be endless questions and as long anyone are in doubt as to their deaths, they would be symbols.
E) "Ruining the royal bloodline" by force marrying them to a commoner. - That would be the equivalent of rape and would cause a huge uproar, probably even among the Communists. Also, Communists too have daughters...
F) Sterilizing them. An unsafe procedure at the time. And it wouldn't make that big a difference. Ekaterina the Great wasn't even in the bloodline, yet she became empress. As political symbols that method wouldn't make any difference either.
- They had to die. The only question was how.
"Sickness", "accident", "suicide", "trying to escape" and so on would be a nice solution. But snuffing them out one by one over a period is risky. They would inevitably try to escape, probably with inside help, and perhaps succeed.
So the logic solution was the one taken: Gunning and bayoneting the whole family down at the same time.