I have been disregarding this very prominent advice since 2015IOW, you've never known a down market.
This is the same lame advice every financial advisor gives. It is even the advice Warren Buffett would tell you.
It's all a conspiracy of the professionals, I tell ya! They wouldn't want you making the same returns they do, now would they?
he invested in individual stocks
Buffet bought/buys entire companies, or at least a large part of them.
I completely reject this notion that investing in individual stocks is some extremely difficult thing to do well.
Not that you'll believe me, but you might very well get your comeuppance. I mean, I hope not as I wish financial ill on few people. But you've had great returns only in a bull market, ignore the advice of professionals who have been doing this for decades, and you seem to think stock picking isn't all that hard. I, who has ridden the ups and downs of the market since the 80s, have seen this time and again where a sudden downturn or recession turns those "models" upside-down. The last one that stands prominently in my mind was the dot com bust. Everyone was "up over 200%"...until they weren't.
Anyone else reading can ignore this. Unless you want to make a hobby out of it, go buy S&P 500 index funds, like everyone tells you to do. Oh, sure, splurge $10K on some TSLA or AAPL once in a while if you're feeling lucky, but keep most of it in the general market. If you do wish to make a hobby of it, know that myself and everyone I've personally know who trades regularly has learned some hard and expensive lessons along the way. It's a good education, and you'll likely be a better investor for it, but it's not free.
(Disclaimer: I trade the hell out of individual stocks. Do as I say, not as I do. <g>)