I don't know a lot about Belted Galloways in particular, and I admit my opinion was somewhat jaded after reading the subject of your post and the opening statement, "We have a belted galloway cross cow..."
Regardless of the breed, cattle that don't perform run a high risk of being more of a liability than a profit maker for a cattle breeder. I'm sure a lot of breeds have SOMEthing to offer the industry, but in the whole scheme of things, there are reasons why a lot of breeds will never be widely popular or useful to the industry as a whole either. If you're serious about the cattle business, then I think you've answered you own question...just read your own post again! If you have to think about this decision, my guess is you're undecided about your commitment to your program or you have some other small niche segment of the business in mind. You listed the reasons you're thinking about culling the cow and her daughters...what are the reasons you're thinking about NOT culling them? I'm kinda curious about those...
I'm not ignoring the fact that a lot of people own cattle--and are entitled to own cattle--who aren't necessarily in the beef business. Some people just enjoy owning a few cows. Some people prefer to cater to niche markets or have their own ideas about doing things differently from everybody else. Still, those of us who do own cattle do contribute to the beef industry in some way, and I believe there are good ones and bad ones in every breed. If you want to own cows, why not own the best ones you can own of whatever particular breed you choose? It doesn't cost any more to feed a good cow than it does a borderline cull.