Someone already addressed all this but I just can't help but really whack these, really should have been clear chapters ago.
Welcome to monarchy where parents often have to be monsters that turn their children into monsters. Did you think monarchies are built on nice words and love where royals get to do or be whatever they want, and everyone loves them for it?
Monarchies are built on iron fisted control. And that control is not automatic, it has to be enforced. Usually that means having the most money and most military power and usually also religious clout to buy off/intimidate a cadre of nobles, who in turn do the same to lesser nobles, and so on until you reach the common folk.
Yes, the same nobles that have plotted to install their own figurehead, are fanatical about hereditary magic because it's the foundation of the above ability to buy off/intimidate, who rule by something equivalent to divine right in this world. The ones that have been described and covered in every single friggin' arc in this story.
Yeah, Annie's parents are the King and Queen? SO? How will they enforce their change in laws? They would erode their own power base by which they would enforce the changes. Changes which DO NOT benefit the nobles, which they have already been clearly opposed to, since the start of the series.
Thats just the ideal. It's doesn't mean its practical, at least not in this generation. Above I'm explaining why but they literally spell this out in the past few chapters so there is no excuse. They can't change it in this generation.
So instead of adopting Euphie, which is the currently proposed solution, you basically want the author to invent a new character who has the required pedigree and magical power for the sole purpose of fobbing the monarchy off to them instead. And it would have to be considerable if Euphie needs to become this demi-god like existence to have legitimacy. And this is a "better" solution solely because you won't care as much about this hypothetical man, probably.
Aren't you trying to fix a plot hole that doesn't actually exist by inserting one of your own?
It does paint an interesting contrast where we as readers want to (and are arguably meant to) sympathize with Anis, because (in brief) she's a magic-less heretical magitec user who abdicated her responsibility, saw her brother burdened, then corrupted, then destroyed by said responsibility, and then Anis is destined for misery as a puppet queen/heir maker, only to be saved by the love of her life who makes the choice to destroy
her own humanity and gain the Crown as a demigod immortal.
But as you said - realistically, the system and setting necessitates ruthless attainment and possession of power, and the monopoly of force held by the noble class means any tension thst escalates into actual violence would see widespread suffering and destruction of the commonfolk who are powerless in the face of the ruling class who have every interest in not giving up their power.
In the long term, things can be changed. Arguably, Euphie becoming immortal makes that easier, actually - except the whole "losing her humanity/sense of perspective" thing. If Anis can keep her grounded while she's alive, that's one thing, but once Anis dies, Euphie's tetherless and growing more alien by the day, making the dream of Anis' kind of a nonstarter because the improvements that would help her wouldn't take root in Anis' lifetime, and once thet
do, Euphie might have forgotten Anis altogether. The only solace is Anis doesn't live a shit life chained to some noble son consort making heirs while having no power, being detested by the nobility as a magic-less royal.
Knowing what I do of the LN plot, some of this is addressed (or being addressed), but at this point in time of the manga's story, it's a weird moment of "solving some immediate personal problems while kicking the can of macro issues down the road and hoping to sort it out later".
I get why Euphie and Anis fought, their reasons for doing so, and why Euphie winning was best for Anis and the country at large (maybe not necessarily the nobles who like having all the power). But the implications of the Spirit Contract on Euphie's connection to humanity, while Anis remains very much mortal, is a powder keg that poses big issues for their vision of the kingdom's future and how it gets implemented in the short and long (and very long) term.