Sherlock I suspect is in the camp that considers Joan a Goddess, although he can't express it yet. How could he live with her for 2 years and feel otherwise? How could he not be deeply in love with her? There have been outward hints of his growing affection; asking if she wanted to search for Australia's Titanic with him, commenting on her transition from Dr. to "consulting detective", doubtlessly a big compliment in his mind. So the thought of losing the Goddess to his brother would be excruciating.
Watson wants a life... and apparently Mycroft! But does she understand the conflict this represents with her role as sober companion? If the thought of his brother being with Joan Watson doesn't drive Sherlock back to drugs, what else could? Does Watson realize this? Great opportunity for future story lines. For me, the relationships overshadowed the whole drone thing in this episode.