The problem is that you cannot be quite absolutely certain that someone will in fact fail. You can express any likelihood of them amounting to anything other than "normal" or "average" is frighteningly small, but that's not quite the same as an absolute fact that they will not succeed ever, nor does any of this mean that the effort to reach their goal on some level wouldn't make them happy even if they never succeed. The effort to reach that goal also can be also very socially and economically productive.

I think the better advice is "Dream of victory, but prepare for defeat." The idea is that if they are truly passionate about something they should push towards it but prepare themselves to fail again and again. That means that they shouldn't just abandon all family and stable work for said goals, but instead maintain those in preparation for the likely event that they fail in each attempt. This is important because no one goes through life without taking a blow so to speak. Everyone spends some of their time taking their own share of lumps and preparing for this instead of living in a fantasy world in which nothing can go wrong is important.

I suppose its a fundamental disagreement of basic philosophy here. You are arguing the Buddhist and Epicurean thought "Unhappiness is caused by unnecessary desire." Whereas my observation and platform is based upon the idea that "True depression is stillness born from a lack of worthwhile purpose and objectives in life." Its the recognition that for some people at least (such as myself) they need fantastic goals and overriding purpose in life to be happy, even if the chance of success is quite low.