Each candle on the following chart of the SPX represents a period of one year.

From 1932 to 1972, it, essentially, rallied for 40 years, particularly for the latter 30 years. From 1974 to 2000, its bull run lasted for 26 years. For the past 10 years, it's also been in a strong bull market.

Very simply, history shows that, for the most part, the SPX has been a strong (BUY) candidate for longer-term investors over an average of 30 years running, and that is likely to be repeated for another 10-20 years, generally speaking.
 

As I write this post on December 28 (with only two trading days left in 2019), the following monthly chart of the SPX shows that it has gained 340% from its March 2009 lows.

The Balance of Power is currently under the control of buyers and has yet to match prior highs...hinting of further upside potential for 2020.

Furthermore, if the 2020's are as strong as the 2010's, look for a similar percentage gain through to 2030...and, possibly again to 2040...before we eventually see a meaningful pullback/consolidation for, perhaps, 10 years through to 2050.
 

As long as the Technology Sector (Nasdaq Composite Index) remains strong through 2020, as I wrote in my post of December 26, no doubt that will bode well for the SPX.

The following monthly chart shows a gain of 554% for the past 10 years. The gains in the tech sector have outpaced the U.S. markets, in general. That is likely to continue and, possibly, strengthen over the next 10 to 20 years as innovation accelerates...buoying the rest of the markets, in turn...one to watch.