idk tho because Eurus killed Victor so then if Victor was John then John would have never existed!<<<no they're not thinking of it literally but in a metaphorical sense. Victor looks so similar to John and must have reminded Sherlock of him.
I LOVE Mary/Sherlock friendship. While John is the most important, I feel sometimes we overlook how much Sherlock needs Mary too. She supports him, supports John continuing to work with him, and helps so much.