Sometimes I feel like when people use the argument that Stannis is “the rightful heir because the Baratheons overthrew the Targaryens…”, I feel like they’re kinda missing the point or at least like, being purposefully obtuse just because they might not like Targaryens or believe that the better a warrior/larger army you have = better fit to rule.
I could argue that just because something is stolen doesn’t necessarily make it legitimate. The same way people KNOW that Cersei stole the throne from Robert, and Joffrey isn’t the “rightful” heir, could use the same logic against Robert and Stannis. Robert stole the throne, he’s technically a usurper, so in that case is there a “rightful” heir? Is Stannis the “rightful heir” or is he just next in line for the throne?
I don’t know, that’s just some thoughts I’ve had, I could be way off base though
So I wrote about what "usurper" means for Westerosi society/simplified EU "feudal" societies HERE.
"rightful heir" is a cultural phrase more to define how, by custom or by "law", "deserves" the throne. "Fit to rule", can overlap with this, but it is not an equal synonym. Just bc you have more soldiers, doesn't mean that you are "fit" to rule, but if you use said army and you won...you are a "legitimate" ruler.
FIRST -- if there had been no suspicion thrown on Cersei's kids, Stannis wouldn't have gone out of his way to try to take the throne and thus he'd be 4th in line behind Cersei's kids...as he technically still is during the War of Five Kings.
SECOND -- The thing with (mostly) successful overthrows of the past monarch is that....this is enough to establish the new, replacement ruling house as the "legitimate" ruling house...bc they won. It's weird, I suggest you read the post I'm linking.
Anyway, Cersei "stealing"--if we read how lines of succession of Westeros are customarily done--would be her "stealing" from Stannis, nor Robert. this is IF it were ever exposed that her kids were not Robert's, which it never was. Usurpations are a circumstance of force and greed or self-interested ambition or self preservation or, rarely, trying to steer the state into an envisioned "future"/"new world" come at force...not black and white "justice". I'm saying the ethical value of the motivation(s) behind usurping a sovereignty are not as moral as people sometimes want to believe or tout/misinform. You find this out while reading ASoIaF, when in the 80s-90s (remember this is when this series was first published) GRRM subverts the tale of the morally righteous ruler who supplants the evil ruling tyrant. Robert could have been the pure-driven knight who banishes the "dragon"...but we question how "good" or fit for rulership he really is when we get to know him and see what sort of priorities he has that are indicative of what sort of hierarchial society the lives and is a beneficiary of.
He regularly cheats on and beats his wife--a juxtaposition of the good, silent, behaviorally "pure" queen consort who supports her husband's every move or draws inspiration from the masses to follow them AND the her motives/relationship w/Robert show a darker, realer side to noble marriages in terms of the inequality. He cosigns on the rape and murder of a woman for power and in hatred for the past dynasty's "dragon" prince that began with a slight to his masculinity. He wants to continue to destroy that last vestige of the "dragon" family, its children (those who are one of the most vulnerable humans), not just bc he wishes to secure his seat but bc he genuinely sees them all, youngbloods and adults, as unworthy of mercy for descuring his sense of self--he dehumanizes them to do so. In a way, he reflects the past mad king in his excesses and malevolence that's only banked by his being more rational. It's even repeated and obvious with how we know Aerys raped and beat his sister-wife Rhaella/Dany, Rhaegar, & Viserys' mother but the Kingsguard didn't do anything bc she is essentially his property and they "legally" can't go against his wishes or prevent him from enacting his will...but rather serve him to carry out his will and preserve his ability to do so by protecting his life above all else unless he says otherwise. Robert, does he not do the same with Cersei, and is Jaime similarly unable to do anything for Cersei without inviting great risk to them both? He is not as unchecked as Aerys, but just as selfish and he still enables bad-to-evil actors around him and they do him AND the quality of that uncheckedness comes not from governmental checks for a protected realm but comes from other nobles moving in their self interests. Nothing really has existentialchanged with his rule expect he has a corrupt council and another person determining his bigger decisions bc how powerful they are, how much they have and can still supply his rule and maintain his seat (Tywin). And we see that partially shown in how a war immediately breaks out after he's killed/dead for the succession.
But despite all this, the "legal"/customary rules riles that he is "fit" for the throne bc he won. He is King/Monarch. Imagine if Dany and Viserys were dead, if FAegon and all the Blackfyres were gone...yes Robert would be rightful and he's established a new dynasty. If Dany comes back or any Targ scion, and they win, he's a usurper and not rightful.
So instead, the question more is: what makes a near absolute ruler "fit" to rule (and before modern scholars come for me, yes monarchies are not great, I'm talking abt the introspection of interrogating current sociopolitical systems...esp those where we see no potential for the rise of peasants or traders or a rise of "bourgeoise" insight...this is a noble-led story, so let off)? It can't just be cultural custom or law. Thus we are divided b/t what the society sees as "fit", how they define it and how rigid or flexible according to circumstance that really ("law" or an individual's actions) is VS what we the reader consider a fit leader. And why do we think so, from what moral, ideologies, or philosophical references do we vs the characters draw from?
All of this works to peel back the veneer of "order" and show just how messy governance can be, can get, etc. how cyclically violent it can be and esp without some sort of existentially higher purpose.
So absolutely, some of them are missing the point (bc idk what else beyong Stannis is rightful heir some say). In fact, they are really just perpetuating & reinternalizing feudal or authoritarian definitions of "deserving", "rightful", "fit", "blood=being", "(male/masculine/able-bodied)warrior = good/fit leader". Because they want absolute answers or a total shift into a new, closer to our own social "democratic" order...which yes people vote, but even the worth of that vote is not absolutely understood or substantial without political understanding and we're back to some sort of "square one".
Fire and Blood serves to highlight that and contextualize Dany through her ancestors and the Dance is a huge old question of "deserving" based on sexism. and to give us more clues-mysteries on the origins of dragons/the nature of the bond. These nobles who could be focusing on more important things and preserve the knowledge of the Others at least...not reject magic and instead maybe learn to use it and their ordinary weapons of war for good, mostly protective purposes. Instead, bc they (Targs [who also "rejected" magic when they rejected their women to assimilate] and other Westerosi houses) are the apex of their society by being nobles and are constantly trying to define lines of power, they fight for revenge and power. They can and do prey on relatively more socially vulnerable people.