I think it's hard for people who like the world to make sense (I have engineer friends who are like this) to deal with markets sometimes. These folks figure if they dig deep enough, there is a formula or a recipe somewhere in the data that will let them make sense of it. The problem is that there's not, of course.
Markets don't do what any metrics or numbers you can find tell them to. They are very very random over the short term and still pretty random in the medium term.
All that said, OP, I usually assume 4% real returns as my base worst case over the longer (20+ years) term. Things could certainly be much better than that (UBI/Star Trek future where we're all FI) or much worse (Covid25 kills everyone, meteor, etc).
This is for the world, which I attempt to invest in about equally weighted with US stocks.
-W