How about the famous quote: "The markets can stay irrational longer than you can stay solvent"

Sane.

"There's too much risk either direction, stay away from it" sounds pretty sane to me.

"Realistic"? Shorting a stock is not a pure mathematical bet that a stock will, in the fullness of time, be viewed as overvalued, it's a concrete bet about prices in the near term. :)

Plenty of people called the 90s dot com bubble, shorted tech stocks, then lost everything because the bubble kept going until they ran out of cash to make short bets. Same with the housing market more recently.

If you could magically know that Company X's stock is overvalued, and that it WILL collapse at some point, but you couldn't figure out when, just a vague "sometime in the next 10 years", going long would be a good way to lose money (it will collapse eventually), but going short would also be a good way to lose money. Timing matters just as much as direction.

A recognition that the market may continue to be irrational for a long time, so even if you believe a company to be extremely overvalued it may be unwise to short it unless you have infinitely deep pockets.

If you insist on a single word, may I suggest "experience."

The one where shorting requires debt and a massive risk premium on the time value of money. Not to mention the unlimited risk piece. The two bets aren’t symmetric

Volatility, or Uncertainty