Oh, no, I didn't say he undervalues her, on the contrary, of he did, the idea of somehow losing her wouldn't bother him. He has great respect an consideration for her, but his first thought is not always for her, it's for how it can reflect on him. If Joan dates Mycroft, she won't be there with him all the time, to work with him, to support him, to be his friend. It's no coincidence in my opinion that we saw Sherlock hiding the dealer's contact, he fears that he may relapse soon. And he doesn't even know that Joan is thinking about getting her own place. I hope it's more clear now