I do not see anything wrong with investing what we can afford to lose.  This simply reminds us that there is always risk involved in investment.  There are cases where investment flops so investing what one can afford to lose in an investment that flops saves him a lot of headache since it will not affect his daily lives financially.

Another thing is investing all we have can end up in not a good situation where we are forced to sell at a loss when needs arise and the money in our pocket isn't enough for these expenses.  Everything should be balanced and carefully planned. So investing in what we can afford to lose is always a safer approach to anything that has something to do with finance.  Btw, my answer to the title is no, the concept of "invest how much you can afford to lose" has never changed.