If a person wears partial dentures but keeps some of their natural teeth, does this mean bone loss and sagging of the jawbone won't be as pronounced?

Partials and bone: In a word, NO. Wearing of partial dentures, even when skillfully done, does little to prevent bone loss and bone sagging. In fact, when not skillfully done, or when the best treatment is OTHER than partials, bone loss may be aggravated or worsened. See a qualified oral surgeon to discuss other options like dental implants, rather than "clip on" partials

Dentures & Bone: Bone will still be lost where the teeth were extracted. Without stimulus, the bone will atrophy. Having partials will help prevent the collapse of the vertical dimension which might be what you are asking. Nothing as good as your own teeth, the next best thing is implants.