In the excerpt you provided from Thomas Paine's writings, he argues that power should be derived from the consent of the governed and that forming a constitution is a wiser choice than leaving the future of governance to chance and the whims of individuals.
Paine emphasizes that it is far safer and more prudent to establish a constitution deliberately and thoughtfully while people have the ability to do so. By forming a government based on a constitution that reflects the will and rights of the people, it safeguards against the potential for tyranny or chaotic upheaval that can arise from discontented individuals seizing power opportunistically—as he illustrates with the mention of a figure like Massanello, who could capitalize on unrest and manipulate the situation for personal power.
In essence, Paine's argument suggests that a structured, consensual system of governance offers stability and protects the liberties of the people, whereas leaving governance to unplanned events and individual charismatic leaders poses significant risks to freedom and order.