This comes up every year and it never gets better as an idea. The women's field is just not deep enough to have the race much longer. On the contrary, the women's race is almost perfect - it is short enough that the middle distance runners can both compete and use the season for appropriate training. If the race were longer either the middle distance runners would not effectively compete or their training would be skewed from what they should be doing.
It is also long enough that the distance runners are in their element (6km XC is much longer than a 5000m track race and the race plays out much differently). To illustrate that 10km is way too long, look at the NCAA field for the 10,000m. Decades after it was introduced the field is still so thin that the runners in 20th-24th are just not in the race. That breaks out to less than one runner for each of the 31 teams at the NCAA XC. Yet, you need a team of seven healthy runners to compete. The attrition rate for women trying to run 10,000 meters is way too high, not that many athletic women can train enough to have deep teams for that many squads.
Yes, it is too bad for those several dozen women that are good at very long distance but not at shorter distances because they have to compete with all those other runners that are not too far from their best distance but that would have the tail wag the dog. It sounds like a really good idea at first brush but, in fact, is actually a pretty bad one.