I guess I have a slightly different take on this than some of the posters above (though I agree with some points):
- I wouldn't do (or encourage a child to do) any EC in order to have an "advantage" for college applications. The enthusiasm to participate in an activity has to come from the heart, and has to come from the individual. I agree with @doschicos that "It's less about exactly what she chooses than that she is engaged in something and that she enjoys it and has a **strong and self-directed passion** for it."
- That said, kids change as often as the weather, and sometimes helping them stay the course with something they generally love can be productive. I never force my kids to stick with an activity, and if they have clearly lost interest in something it's good to move on to other things; but I do encourage them to stick with that they obviously enjoy and in which they have invested a lot of time and effort to develop proficiency - not because it will give them an "edge" for college, but because sticking with something to the point of mastery is a wonderful thing, and yields all kinds of dividends for life in terms of self-confidence, sense of achievement, ancillary skills, etc. If they'e truly lost interest, then you move on; but if they've just gotten temporarily stopped - by boredom, frustration, or the transient lure of other things - then it can sometimes be helpful to keep them on track. That can be a fine line.
- The intersection of art and STEM - "STEAM" - is one of the hottest areas of education and technology. Merging visual and design skills with technical skills opens a huge number of doors. Computer game design, animation, data visualization, design fabrication ... there are a ton of "mainstream professions" where people with STEM and visual skills can excel. Penn Digital Media Design grades reportedly make more coming out of school than do Wharton grads. I agree with @gouf78 that "Art is the ability to see things in new ways and create a vision and science allows you to bring a vision to life.Someone with training and aptitude in both will be highly sought after whether they lean more towards the art side or the science side."
In the case of @SculptorDad’s daughter, it’s not clear to me what the situation is. She clearly had a strong passion for and aptitude for art, and had invested a lot into it. If she’s lost interest and moved on to other things, that’s fine; there’s nothing wrong with interests changing, and I certainly wouldn’t pressure her to continue with art if that is the case. But I’m not sure I agree with @happy1 that “it sounds to me like her interests have changed and evolved”; to me it seems like it’s possibly more a case of going with the flow in terms of the opportunities and interests available to her at her boarding school, where STEM is strong and art is comparatively weak. It sounds like art is something that she wants to continue for life - just not as a profession - so she hasn’t really lost interest in it.
If this were my child, I’d probably try to figure out whether they’ve actually lost interest in art, or are just excited about other things and have a more “mainstream profession” path in mind. I certainly wouldn’t pressure them to continue art as an EC that might give them an edge; but I’d point out the opportunities that are available to people with combined art and STEM skills, and see if there was interest in continuing art as a complement to STEM, and looking into ways to blend them. I’d look at the profiles some recent students who have blended art and STEM and see if any of those paths are exciting. The choice obviously has to come from the child and not the parent, but pointing out that that art and STEM aren’t mutually exclusive may help. After that, I agree, you should try to be happy with whatever she chooses, and find her path at her own pace.