I see what you mean. The fallacy lies in the assumption that people are naturally good. The closest I can think of is false premise fallacy, since you say they're basing their argument on this belief.

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Bandwagon Fallacy The bandwagon fallacy assumes something is true (or right, or good) because other people agree with it. A couple different fallacies can be included under this label, since they are often indistinguishable in practice. The ad populum fallacy (Lat., “to the populous/popularity”) is when something is accepted because it’s popular. The concensus gentium (Lat., “consensus of the people”) is when something is accepted because the relevant authorities or people all agree on it. And the status appeal fallacy is when something is considered true, right, or good because it has the reputation of lending status, making you look “popular,” “important,” or “successful.” For our purposes, we’ll treat all of these fallacies together as the Bandwagon Fallacy. According to legend, politicians would parade through the streets of their district trying to draw a crowd and gain attention so people would vote for them. And whoever supported that candidate was invited to literally jump on board the bandwagon. Hence the nickname “Bandwagon Fallacy.” This tactic is common among advertisers. “If you want to be like Mike (Jordan), you’d better eat your Wheaties.” “Drink Gatorade because that’s what all the professional athletes do to stay hydrated.” “McDonald’s has served over 99 billion, so you should let them serve you too.” The form of this argument often looks like this: “Many people do or think X, so you ought to do or think X too.” One problem with this kind of reasoning is that the broad acceptance of some claim or action is not always a good indication that the acceptance is justified. People can be mistaken, confused, deceived, or even willfully irrational. And when people act together, sometimes they become even more foolish—i.e., “mob mentality.” People can be quite gullible, and this fact doesn’t suddenly change when applied to large groups. Which of these is a bandwagon fallacy? Example 1: “Almost everyone at my school will be at the party Friday night. It must be a popular thing to do.” Example 2: “Almost everyone at my school will be at the party Friday night. It must be the right thing to do.” https://thebestschools.org/magazine/15-logical-fallacies-know/ EDIT: oops Know_Thyself8 stated it. More on reddit.com

For example, at one time everyone believed that the earth orbited the sun, but it turned out that everyone was wrong.

typo

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Good list, but needs actual examples of the fallacies DDS say. Bandwagon Fallacy: "16 million people can't be wrong." Non Sequitur / False Dichotomy: "Either you support EJK or you are pro-drugs." Red Herring: "E pano yung mga rinape ng adik, wala ka bang malasakit sa kanila?" Ad Hominem: "E dilawan ka pala e. Tapos na tong usapan na to". Appeal to Authority: "Nabasa ko sa Mocha Uson blog. Malamang totoo." More on reddit.com