I remember reading that even if you wanted to avoid doing that quest she's still getting uncaged in the epilogue. And I always thought then why not just stick heir what not in the epilogue too. Who knows though. Maybe you can safely opt out. No use being mad about something not implemented.
No. And there's probably never going to be a reason. Soulbinding is a dumb concept given the games developmental track record. None of the choices are ever going to even remotely receive anywhere near the quantity of content that Kassyra is going to have. Both from a gameplay perspective (the powers all kind of suck) and a raw scene thing, given that Kassyra is Savin's baby next to Cait and between the two of them they're going to have the lion's share of content, sexual or not, of companions in this game.
You'll get maybe 1-5 scenes and lose out on literal entire expansions worth of character development going forward completely exclusive to her romance path. Like visiting her in the ways between, the talks, etc.
Nothing about the current character writing requires reaches or imaginations from anybody. Even at high libido she's still a subby 'cock sock' except now she spreads for other people. Before they "fixed" it this included multiple other NPCs with no input from the player.
There's no foreshadowing here for what's coming in the future. It's not like how we know how Kasyrra and Tollus, in relation to the soul that is, are eventually going to dance as that's foreshadowed in multiple spots.
This is further aggravated by the game itself. You've got the what not in, that ghost quest that talks about heirs
You've got the codex that talks about the sanctity of marriage
So if you read the lines, in between the lines, through the lines, over the lines, around the lines, they all point to a pretty seeable conclusion. MC likes (or maybe is 'indifferent to' in your playthrough) Ryn. Ryn likes MC. Potentially after everything is over they'll probably shack up.
The heir stuff is a pretty big surprise (or will be for the people unaware that it's coming, at any rate). Moreover, depending on the person, it's going to be a rather unpleasant surprise indeed. I'm a bit curious to see how much pushback, if any, there is if she bangs Elthara and she gets as lovey dovey as Elthara does with Brint when she's carrying their kids. Would be rather shocking for you to be for a character every step of the way, fulfill their dreams and aspirations, and the end result of a quest, dungeon, character development arc, and years of waiting is for her to immediately go bang somebody else. Honestly I feel bad for some unaware male-MC shmucks who have one-itis for Etheryn. They're in for a rude (though admittedly funny) awakening.