Risk is a very hard thing to properly assess, even if something...

Risk is a very hard thing to properly assess, even if something has a 80% chance of success and a 20% risk, you cannot put everything on it unless it is your first bet and you have nothing to lose (starting career, no mortgage).

The 20% chance is not a chance of a 20% loss, it is a 20% chance that the risk is realised, and the realisation of that risk could be a 100% loss of capital. So, if you lose such a bet early on you have your entire working life to recover, a favourable performance of +1-2% per annum will easily cover that, none the worse for wear.

But halfway through your career with 1/2 to 2/3 of a mortgage, going all-in or even significantly overweighting a speculative investment could result in a significant loss, perhaps an irrecoverable one. Say the 20% risk of failure was realised by someone aged late 40s to mid 50s, they will have the unenviable choice of accepting that if they start choosing "sensible" investment options from now on, that they will only ever achieve a below average outcome in retirement. Effectively losing years of work in the market. Years of work, all those hard days, nothing to show for other than a guaranteed below average retirement.

If that outlook is too difficult to face, such a person could feel that the only way to recover an average outcome after this loss is to take on more risk. We all know that temptation is irrational but it speaks more loudly to us in the dark night of the soul. Down by half, double or nothing. Younger me might take that approach for courage, older me knows that approach is madness. I don't want to be in that position.

So, my take, maybe DM should have left the free carry advice to a CFA. But, unless you are starting out and have nothing to lose then free carrying and taking some profit is sound advice and worth considering regardless. We don't need any encouragement to think about profits and bags, but it is worthwhile to really appreciate what we have on the line and if we can truly "afford" to lose it.