Right to rule was barely a mechanic; and has largely been replaced by clan tier which achieves the same purpose (A checkpoint before starting a kingdom).
RtR - Softcap on kingdom creation (as making a kingdom too early would cause non-stop wars). Raised sharply by a few game actions.
Clan Tier - A hardcap on kingdom creation. Slowly raised by pretty much all game actions.
The only difference between these two is the soft/hard degree of the cap and the fact that clan tier has other mechanics built in (like companion slots). They serve the same purpose - to extend the midgame.
And as far as warband goes - there was no diplomacy outside of peace/war (the occasional flavour event). I don't disagree with your statement per say... but worth mentioning that Warband has the exact same issue.
I mentionned those mechanics as elements that should have improved from Warband.
Right to rule was simple enough, and though it was minimal, it worked: you needed to have gathered support before having a shot at creating a kingdom. The critical difference with Bannerlord is that high right to rule decreased the chances of getting gangbanged by all factions right from the start, which is not a thing in Bannerlord.
I do agree that clan tier could be used in a similar fashion, though the fact it applies to clans and not characters makes things more complicated.
What I don't like about clan tier is that it's necessary for everything, utlimately becoming an almost forgettable aspect of the game, you will increase in clan tier no matter what. On the other hand, you could lose right to rule in Warband, (though admittedly very rarely) which made it more of a dynamic ressource, like influence now.
There were alliances and trade agreements, which is already much more than in Bannerlord. Having the certainty that a neighbouring faction wouldn't attack you (non-agression treaty) and could even support (devensive alliance) you was very useful and good for roleplay. Plus, other factiions did this too, which made world diplomacy already much more nuanced than War/peace.
Anyway, back to the Minor Factions. I referenced Warband Right to Rule because it would make sense for Minor Factions gameplay: gather support around Calradia so that the Minor Faction doesn't get crushed right away being seen as unlegitimate. Clan tier is nice, but it doesn't seem to have that functionnality. So how do you prevent endless war? Appart from fighting the lords over and over, and executing as many ennemies as possible?
You don't. Hence why those elements are very much needed for a Minor Faction playthrough to be interesting, which is also why it will never happen.
I'm mad now