Taking a step further, if the last thirty five years are any indication, that future $21k would be worth less than today's $10k.
Besides, to overcome inflation, you'd need to average double digit returns on your investment every year for half a lifetime.
Like you say, it's a tough decision if there's anything that can provide you value now. Not to argue against savings, but expecting it to grow exponentially with no effort is folly.
Funny how a mistake in a single sentence earns vitriol on the entire comment.
Despite what I'd mistakenly wrote, I meant that to overcome inflation and see a return of double to quadruple your investment - which is what the comment starting this thread suggests as the outcome - you'd have to beat the market by around 10%.
Regardless, my point was more to do with whether someone with only $50 to spare a month is truly in a position to invest in anything or whether they might be better off saving it for a rainy day or something like that.
If someone has a few dollars to spare come month's end, but has found themselves skipping the odd meal, that money would probably be better spent on a small grocery trip than putting it into an ETF that'll take years to turn a profit.