Well, there's at least one narrative reason for Mathilde to take an apprentice, Eike or someone else: The colleges kinda require it.
Like, I don't know when, but at some point Algard would have to start nudging Mathilde to take an apprentice so that her knowledge and skills can be passed down to a new generation. He'd probably be polite about it, because a) Mathlide is a busy person, and b) she's earned a lot of goodwill from the colleges, but soon or later she would be expected to teach.
Besides, I trust Boney to do it justice. I mean, I really, really, really didn't want to do the waystone arc at all (and then, when it was chosen, I wanted it to be based in Kislev). But I'm not mad that I lost those votes, because Boney took something I had very little interest in and made it interesting, and I'm actually enjoying the waystone arc because of his skill as a writer.
When it comes to having an apprentice, I trust Boney to also make that interesting, and it'll be a shift and a change in the story.