Can crypto currency be used easily? Yes, through centralized entities like where your parents currently store their money.
Some people may forgoe ease and aim for self-custody because they value decentralization over convenience. Others will choose a middle ground where they and a centralized entity both hold part of the key, ensuring that the 3rd party can't move their funds without permission.
Cash has been rejected by most people due to its poor security? I have to ask for a source on that. Cash is still the most popular form of money, and if you have low trust in your government or institutions/live in a corrupt society, is more secure than other options.
Some people may forgoe ease and aim for self-custody because they value decentralization over convenience. Others will choose a middle ground where they and a centralized entity both hold part of the key, ensuring that the 3rd party can't move their funds without permission.