Both Sarah and Alex are two main and highly important characters, and if their character, for any reason, fails to meet players' standards, the game loses its appeal for them. However, if we were to choose between the two, I personally consider Sarah more important because there's more sexual tension and excitement in her relationships with other men than in Alex's relationships with other women. Still, I emphasize that this is a comparison between what is "important" and what is "more important," not between what is "important" and "unimportant"
Regarding the second part of your statement:
That's correct; when there are too many separate paths, the game turns into a collection of short, shallow, unfocused, and less serious stories. So far, the developer has managed to progress through the three paths—Loyal, Sharing, and Netorare—without this problem arising, and we've had a cohesive and well-organized game