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Which is the main - or perhaps the only reason - why some countries have due process in the first place.
It is not that social elites just decide to extend it on everyone out of grace
It is that those in power want to extend it on themselves, so that they could not be killed, jailed or exiled extrajudicially, just out of political expediency
This is a naive surface level conclusion. Ask yourself why. The answer is because the nobles, or whoever matters in your example society, isn't gonna have much allegiance to a system then are basically disposable to and will be disposed of at the drop of a hat and so their allegiance will be just about the same.
A system where nobody really supports anything beyond the degree to which it keeps their head on their shoulders and everyone is looking over said shoulder is gonna have a lot of disruptive power transitions (in a "you go bankrupt slowly, then suddenly" sort of way as people all throw their weight behind the new thing as it starts to gain the upper hand) and engage in a lot less long term productive activity than societies with due process or some other way to keep people from losing everything at the drop of the hat. And it will get out competed by those societies.
Those scheming up evil ways to levy ruinous fines for failing to use one's blinker or get people kicked out of industries for having odious opinions on unrelated subjects ought to take note.
I believe due process is important in every case, and I want to believe that having a mixed system eventually results in due process being skipped more and more, but the example from the end of the article goes against that: England and France diverged, so it’s possible for a mixed system to go either way?