He doesn't own Marvel, but he's in a position of power at Marvel, and he's doubtless not the only person who thinks the way he does. Company culture encourages selections for hire that fit the culture.
Power set isn't a real barrier unless people at Marvel choose to pretend it is. We have a billion different psychics because past writers acknowledged the power set doesn't require only one psychic character getting good use. It's not down to if they have the same powers. It's down to how they use those powers, and who they are as characters.
That said, I would complain if they suddenly introduced another mutant that can use electromagnetism. Because Lorna still needs a lot more from Marvel. Introducing such a character would come off to me as saying they have so little respect for Lorna that they want to replace her with this new character. Magneto wouldn't be hurt in such a situation because he's too iconic after decades of heavy promotion, and Marvel's proven their respect for him repeatedly. They haven't done the same for Lorna.
No children is good because Marvel hasn't done enough for her without them yet. "Keeping a man" is a problem with Marvel, not the character. Not going off on her own is a problem with Marvel, not the character (and ironically she's gone on "journeys of self-discovery" a few times, but so far only for Marvel to throw her into limbo). She's led teams as a fill-in leader, and as ANXF's intended leader, but the lack of her doing more as an intended leader is again a problem with Marvel, not the character. I addressed power set above.
The premise of what I quoted is basically "Marvel's treated her bad and that's just the way it's always gotta be, may as well sit back and watch Marvel keep sucking." I don't buy that premise. I expect Marvel to do and be better than their past. They can't be perfect, but they can make a good well-intentioned effort based in respecting the character and what fans see in her. And by fans, I mean people that actually like Lorna, specifically, for her potential and everything she has to offer in and of herself. Not people who are fans of a certain writer or era, or fans of a different character that's benefited from misuse of her, or similar situations that boil down to being "fans" of how she can be used for ends other than what's actually good and fitting for her.
I shouldn't feel like I could do a better job of writing Lorna when I read a comic with her in it. I should be awed at Marvel doing amazing things with her I never imagined could happen, but which fit her perfectly. I should feel like they understand her essence and are taking me on an amazing journey through layers I didn't even know she had. The Kris Anka concept of a modernized costume for Lorna wowed me when I saw it because it did so many great things with her visually that I didn't think could be expressed visually (minus the Magneto bubbles). I want that from the writing. But the writing keeps missing, because it keeps refusing to make any real attempts at understanding who she is and what she's been through. It's like looking at the Obama presidency and stopping at "He wore a tan suit once, let's focus on the tan suit." Why should I give Marvel any money if I feel like I can do better?
I think Duggan's run so far is a good start at Marvel doing well by her. It's doing good things with her I wouldn't have considered, but which fit perfectly. Now they just need to do it regularly, instead of once every 20 years. Not that there isn't still plenty left that can and needs to be covered. But it's just getting started, and it needs to start from somewhere.