Basis for equity optimism in one chart?

I got this chart from more than one source, so it tells me that it has been making the rounds of analysts. The argument is that the stock market has been on a long-term super-cycle and the trend is always upward. Be long stocks. There may be sideways movements which may last for years, but the those are just times to prepare for the next upward leg. 

I don't believe this, or at least I don't think this is a good argument to always be long the market. Equities should be a core holding, but those periods of consolidation should not mean sticking to the same old allocation plan. All of those sideways periods were times of economic upheaval and overvaluation. It just tells us that times change and we have to change with them.