I don't disagree with you about Jack or the other young or newer characters necessarily, however Seven's story isn't done. I don't see her becoming captain as the end of her story. It's her reconciling with her humanity, or balancing her Borg and human halves, that's what I see as her arc. And now there's a lot to play with regarding what Seven will be like as a captain. How well will Starfleet embrace her and so forth. How will things turn out with Raffi, how will they work together, etc.? And there's a lot of Voyager reunions left, as well as catching up with DS9, and even dabbling in the other new Trek series for world building.
Further, Jeri Ryan and Seven have always been popular, and I can't see them pushing her to the side for Jack, a relatively new character and even though he's the son of Trek royalty, and made an impressive debut, I don't see him alone as carrying a series just yet. If this was say a Jack and Beverly, or whomever, on the frontier spin off, that's one thing, but if they do a series set on the Enterprise, with a popular character as the captain already, I can't see them not giving Seven the spotlight, or a very sizable chunk of it. So far, the only Trek series that I've seen successfully get away from the captain as spotlight is Lower Decks, but even that series gives screen time to Captain Freeman, and perhaps the reason they made Mariner her daughter was to not go too far away from diminishing the captain's role. It's one of the tropes of Trek. Discovery tried it, but ultimately, they reverted back to what has become expected of a Trek series.
I also can't see them not making Seven the main face of Legacy when it comes to promoting the series. And it would be strange to have all these live-action captains as the faces of their series, and then Jack.