By "legitimacy" I don't mean that people agree with the conclusion being expressed, but that people agree it's at least an attempt at a structured argument based on actual facts. The 95 Theses were complaints against real injustices, not a set of fabricated smears.
Colonial independences generally were discussable - and discussed - at the time and in advance of their happening.
Not in the way you're thinking. This is at the time when the King had absolute or near absolute authority. King John was the ruling class. While it was the English nobility that opposed the king, they did so (1) at great risk to their own safety, and (2) as the only ones in a position to do so. Certainly no serf could have accomplished this -- they would have been killed.
Second sentence of the (excellent) Wikipedia article about it:
> First drafted by the Archbishop of Canterbury to make peace between the unpopular King and a group of rebel barons, it promised the protection of church rights, protection for the barons from illegal imprisonment, access to swift justice, and limitations on feudal payments to the Crown, to be implemented through a council of 25 barons.
In theory, perhaps. But the barons could and did challenge the King with a degree of success. The king did not have a practical monopoly on the use of force. So the description of Magna Carta as an 'agreement among the ruling class' seems reasonable.
This is far easier said than done. Just for starters, which "people" get to decide, and that's not even getting into the messy details at analyzing any arguments or facts.
We know who gets to decide - the owners of the communications platform and those who influence them, under current democratic legislation, which is completely subject to change according to the will of the citizens of a democratic country.
It's interesting how easy it is to observe stereotypical right-wing pedantically technical justification behaviors be adopted by the left (feel free to correct me if my assumption is wrong) as the topic of specific discussion moves around various dimensions of reality.