Indexes of SP500 without Boeing should outperform ones including Boeing, if you’re predicting that Boeing will underperform in 25 years. You don’t have to short; you can use the knowledge to tune your retirement holdings.

You could even weight the index ratios by how self-cannibalizing you believe each company will be.

Requires a broker that does fractional shares or a lot of money, though.

You can buy options (puts and calls) for three years out.

Shorting a stock for that long would be a recipe for disaster, at some point it's going to go up and you're going to lose it all. Remember when you buy long your positive upside is infinite and your downside limited to 100%, when you short it's the opposite.

Sure. Just hold a small short position.

As they say, you’ve gotta be careful. The market can stay crazy longer than you can stay solvent.