If you find yourself going down the wrong road, the sooner you can turn yourself around, the sooner you can get back onto the right track.
Your sibling has "buyers remorse". Whether or not you or I agree with their reasoning, they're probably not going to want to ride that bicycle very much. The sooner they come to grips with that reality the better off they are going to be.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.