Looks like this arc is going to directly confront the fact that none of the current club members are all that Mystery Interested either. It's definitely not a coincidence, given how their intros are framed, that the old juniors are all very visually similar to the current juniors. The first three shown are obviously the main cast even being shown in the same order they were introduced to the story, but even the other three look a lot like the three girls that Iris made friends with in her class.
The difference, as others have said, is that the current three Juniors are willing to meet Yuriko Half-Way. Toiri in particular, given his general personality and his reactions to some of Yuriko's more outlandish mysteries, is still not all that personally interested in mysteries, but he's willing to play with her because its obviously something she cares about. Which is a remarkably healthy way to approach different interests in a relationship. He's willing to learn about her interests, while at the same time not trying to change or appear to change his own interests.
He's perfect for her, now and later in life. He's just a rather ambitionless Well-Studied High Schooler, so he'll be able to easily get a normal Office Job and make good money reliably, giving them a steady source of income so Yuriko can focus on writing her Mysteries through the initial period where being a writer writing what you love probably won't be getting you a lot of sales yet.
RIP Tomboy and Lily.