LMAO, so many people inserting themselves into Yamaguchi.
Not sure why everyone thinks Yamaguchi is 100% right, though. Satou is an easy target, but that doesn't mean there's zero chance she can find herself a good pen-pal and stories of pen-pals growing closer aren't unheard of. Yamaguchi's worries are understandable, even more so considering Satou, but he really was rather insensitive about this. And i kinda don't feel like that person is a threat to Satou, tbh (i mean, it's the second time they failed to meetup now - i'd rather think he's avoiding meeting up).
Also, an unpopular opinion - Satou is cute and all, but i can see how people can see a naive slowpoke like her annoying.