Yeah, in the last decade or so, the S&P index was an obvious choice for beginners but not only. It yielded a really solid return on average, and was definitely a better option for new investors than attempting to cherry-pick a particular stock.
The problem is that people like to think that if some instrument performed well in recent years, the same will be true forever, but that kind of thinking can turn out to be a very costly mistake. Nasdaq is a good example. After a sharp peak in 2000, it tanked badly, and it took 15 years (!) for it to return to the previous ATH, and that's without even accounting for inflation. When a product becomes overhyped and too popular, the risk of such thing happening increases.
And, on top of that, we have to take into account unprecedented times of economic war between the USA and China (or even the rest of the world), with a big chance of the US eventually losing its hegemonic position.