Of all the possible complaints about YouTube demonetization, complaining that you don't meet the objective criteria for how popular your channel must be is surely the least important.
If you have a financial arrangement with any company, and that company decides to change their policy arbitrarily so that they can justify not paying hundreds of thousands of people like you "because our policy excludes them", when a day ago they didn't, normally that would be various immediate court cases and no way for that company to win on any reasonable ground.
This is surely not the least important reason.
This is false. If money is being exchanged for services or (digital) goods, then there is a financial arrangement, in the form of an informal contract. But it's still a contract.
In the case of YouTube, it would probably even be considered a formal contract since there are the written "Monetization Criteria", "Partner Program Policies", and YouTube "Terms and Conditions" that the creator agrees to. This agreement to written requirements is a "meeting of the minds" and thus constitutes a contract.