Other people have covered how the Dow is a mediocre stock index. The other half is that 20k doesn't mean anything either. It's just a number calculated by the math-function of the index.
The psychology is mildly fun, just like a 50th birthday may be more celebrated than a 49th. Round numbers appeal to us. But 20k dow says nothing more about the state of the economy than the 19.9 dow did a few days ago.