I guess historically, there was always debate in kingdoms about inheritance. Powerful lords always wanted a say. Weak kings couldn't always prevent that. Powerful kings wanted their kids to inherit, but sometimes their kids weren't capable of keeping power. There is no standard.
Often warlord societies could be elective in nature - for example, many early English kingdoms were very much elective - in fact England was nominally elective until the 18th century - even though by the early middle ages it had essentially become blood inheritance in practice. The same is also true of many steppe societies - the famous example being Mongol prince Batu's withdrawal from Europe to join the leadership election. The Holy Roman Empire was another famous elective monarchy, but again, the elective element became ceremonial over time. Heirs were often chosen and elected from within closely related ruling families - and by the middle ages, all the most important families were connected to the royal line, and even when these monarchies were elective, plenty of legitimacy was still gained from being anointed heir by the previous king. Over time as power and wealth was closely kept by one or two families and nations became more bureaucratic these elections became more ceremonial in nature rather than actual elections, or they were dropped altogether.
So I don't see any problem with elections for successors to kings in Bannerlord. Although variety would be a fun way of differentiating factions. But that's a whole lot of coding.