Green - Linear Regression 2009 bottom to present. Unsurprisingly, been brawling there repeatedly. 4270
Aqua dotted - Regression from 1932 bottom to the present day. 3200
Purple dotted - Regression 1996 to present. 3000
Various fibs coming out of the Great Financial Crisis agree with 3000 more than the others in my opinion. Also in more recent history the market did spend from 2018 - 2020 gathering enough steam to bust out of 3000. Lots of activity there so it means something to a lot of people.
Caveat: One could argue the entirety of the price(s) action in this index was heavily influenced by low interest rates for the last 40 years. That perspective might allow a viewer to question the validity of any analysis if it is known that condition is no longer valid.
Slap the -2SD on 'er if you want a darker forecast.
Bonus Chart - I just slapped Bollinger Bands on the monthly for a quick take on the whole SD thingy. 3500 down there would be what all of our friendly neighborhood computers thing is fine for variation and would not be a 'crash'.
I guess if we didn't get a violent reversal at that level, then we can start thinking about a real event. Big damn hurdle though,we'll see...