Can scaling and root planing always result in lower gumlines, leaving the gums healthy but unaesthetic?

No. Scaling and root planing does not cause the gingival margins to recede... Periodontal disease does that. Scaling and root planing rids the teeth of hardened deposits of calculus. As the inflamed, puffy gums heal and tighten up to become healthier, they may recede towards the bone.