You are not putting them all in Bitcoin (all of your effort your entire life), the idea is a temporary all in where your free money will go to Bitcoin as investment. If you really read what op wants to tell us, this is not for people who already have financial responsibilities to their family instead this goes to students that are still dependent to their parents. So as I said this advice is not for all but for those who are still dependent to their parents with no financial reponsibility. And this is to hold BTC at a long time not just to daytrade it.
Go all in on regret later. You are on a privileged position to profit from high risk high reward moves. No risk no win, and BTC is the best long term high risk bet.
But think about it, if you are still dependent then that means you're actually not yet earning that much too. If you invest everything then that will be just a small amount compared to young professionals earning a living already. If for example, you are a student, you just live off your allowance and you save around $100 a year and invest that all in bitcoins. Prices go up by 100 times by 10 years, that means the value of your money is not enough to retire.
Well at least you can have a good start up. The point here is, rather than kids spending their money to waste, it is better if they save it in form of BTC. This way they savings have a chance to appreciate in price value. If we are talking about young professional, their case is different since they are now tending to themselves and already have financial responsibility. So the approach on how they have to get Bitcoin is different. And I agree that they (young professionals) should not go all in to Bitcoin.