I mean nothing more than that if you were to put the S&P 500 graph over the last 15 years and extremely naively look at it, you'd conclude it was very high at the moment. Naively it would seem that it will see 1600 before it sees, say 2500.

Also if you look at long term history you'd conclude something similar : the US has had a very long (if somewhat disappointing) economic recovery since 2008. We're due a bust.

And finally, the playbook from earlier recessions does appear to be unfolding : manufacturing production has dropped by a lot, and services has flatlined. This is very much like the start of previous recessions. Next up is bad loans to commodity producers (ie. miners) and manufacturers actually going bad (meaning banks declaring losses), then a few defaults, then panic, then recovery.

I would expect index investments made today will seem like bad investments in 10 years. I would never dream of predicting 50 years out. To be fair, I would have told you the same a year ago (not about the recession playbook, but probably everything else listed in this post).