The argument "Eike could get her own better masteries if we don't teach her" feels like it's just gaming the system to her detriment. This isn't rolling a gacha to see what pops out, it's the responsibility Mathilde took on to teach Eike what she needs to survive on her own as a Journeyman.
Like, not to give Regimand guff for being more a father figure than a mentor, but Mathilde was really not prepared for the world and ran into situations almost immediately where she could have just died for lack of necessary skills and caution. And while Eike seems like the type to have a more 'normal' Journeying, the conspiracy just goes to show nobody gets to choose where their Journey will take them.
This is an excluded middle argument. You can easily hold the position "we should prepare Eike more for her Journey than Regimand did ours" (though I will note that Mathilde seems to have been a bit of a poor student, cf "she straight-up
forgot most of her Petty and Lesser spells")
and the position "we should not seek to pass on our Masteries to Eike." There are so many things we can do to prepare Eike for her Journey that don't involve Masteries! We've done a bunch of them and we're doing more!
I'm fine having Mathilde teach Eike Shadowsteed eventually, because I agree with
@Alliterate that
narratively it makes most sense for Eike's first true spellwork of Ulgu, touching actual shadow magics and not just Ulgu's take on spell effects that any Wind can do, to be under the personal attention of her Master. I think that's a great scene idea and I want to read it. I just agree strongly with
@Parabola that I really want to see what relationship Eike develops with Ulgu of her own, not just adopting Mathilde's paradigm, and so I don't particularly want to actively try to pass on our Mastery. If it happens, it happens, but I'd rather leave it in Ranald's hands (like Aethyric Armour was) than have Mathilde be the sort of person who
deliberately tries to make her student do things the same way she does them.
(Codifying Shadowrider as a separate spell in its own right would be a different matter, because that involves translating the Mastery to the "universal" Ulgu paradigm, rather than being reliant on Mathilde's specific paradigm. That's totally fine, because it doesn't involve other people being required to adopt our way of thinking, which is the thing I object to.)