Just invest in the S&P 500. It isn't rocket science.
The S&P tripled in value since 2010, so you would've had to invest $333.000 at that time and cash out now to become a millionaire. But few people have that. That's a 1%, I already made my fortune / I have rich parents privilege.
Sure, ANY amount you put in there would've gotten tripled, but turning $1000 into $3000 isn't going to make that big a difference.
And if you have hundreds of thousands lying around, I'd argue that buying a house would be more beneficial for your well-being in the short term.
Disclaimer: I've invested some money in the past, both low-risk long-term stuff, index funds, and 'play money' into meme funds like Apple and Tesla. I think at most I've doubled what I put in? Didn't lose money (even with the rona), that's for sure.
I try to not go "I should've done x" too much though, like "I shouldn't have sold my handful of $250 Tesla stocks when I did".
If you'd done this 40 years ago, the million dollars you'd have today would have the equivalent purchasing power of $300,000 in 1980. Inflation is a cruel master.
Some common answers might be:
* healthcare, always expensive
* things for your kids, including education, housing, access to jobs
* random consumer spending including holidays or travel
* charity
Presuming you were able to actually afford saving $400/mo while living reasonably comfortably which is definitely not a given and not quite as common as expected by some.
And presuming you're not altogether unlucky in your strategy, which can happen.
You will have probably also accrued still useful assets to pass on or use. Or lost them.
$400/mo saved in a 0% interest bank account for 40 years = $192,000
$400 * 12/year invested at 10% (incl. inflation) for 40 years = $2.12 million
While $2.12mm in 2020 money is $440,000 in 1980 money (assuming 4% inflation), it's still much better than the alternative.
A couple of other points:
* Investing $400/mo earlier is relatively harder than $400/mo later in the 40-year timeline assuming compensation increased by the inflation rate but one keeps the invested amount ($400 here) constant. This is ignoring factors like salary increases later in one's career (not sure if that even generalizes for lower-income professions).
* I wonder if there's any study done tracking if there's actually a steady increase in spending consistent with the inflation rate for all individuals (with some stochasticity). Or, do people adjust the goods they buy and "outwit" inflation? For e.g., the biggest purchase one makes usually, a house/apartment, doesn't scale with inflation. Similarly, electronics actually get cheaper. So, apart from food, does spending actually track inflation?
VT is another good choice if you're lazy and just want to own a slice of the global stock market.
To put it another way - it rolls off the tongue better than "buy a weighted amount of a CSRP (or comparable) US Total Market Index and FTSE (or comparable) Global All Cap ex US Index, where your weighting is probably slightly overweighted to domestic".
Often (usually?) when people say it, they aren't literally advocating to only buy S&P 500. Some people still do recommend this, but I'd wager they're closer to a minority now. Famously I'm pretty sure Warren Buffett is still advocating S&P 500 only, and it takes a long time for voices like Michael Burry to outweigh. Vanguard pushed out a whitepaper years back, and made a very public change in their own corporate 401k to get rid of the s&p500 fund choice.
2. SP500 companies earn ~40% of revenue from outside the United States