Haven't these so-called experts been predicting "much lower future SP500 returns" for decades, now?
With regards to the 2000s being a "lost decade," I did a really simple calculation. I assumed starting with $100 on 1/1/2000, and then multiplying by my actual annual returns for each year. By the time I got to 12/31/2009, I was at around $98.50. Ouch!
However, a good chunk of the decade was actually pretty good. It's just that 2008 really knocked it down, and then 2009 built it back just enough that it almost zeroed out.
For that decade, additional investments accounted for about 78% of my asset rise, compared to about 22% of actual market gains.
In comparison, 2010-2019, additional investments were only around 29% of my asset rise, versus around 71% being actual market gains.
From 12/31/2019 to now, it's about 25% additional investments, 75% market gains.
So even though I did manage to make some money in the "lost decade" thanks to dollar-cost averaging, rebalancing here and there, and perhaps some accidental market timing, that decade pales in comparison to the 2010s. And, so far, the 2020s.