Depends what you believe will happen with the markets and inflation over the next 15 years. I believe a better approach in today's world is to put yourself in a position where you're less dependent on historical market performance.

You are free to believe that. Historically, the stock market has returned 7% on average. That is after inflation and takes into account the several terrible crashes that have occurred.

Do you know why the stock market has returned 7% on average? Or do you just think that, since it returned 7% on average in the past, it must do so in the future?

The U.S. has held a very privileged position in the world economy over the past century, which has fueled tremendous growth. You're basically betting that the U.S. continue to be a "growth" stock over the next 25 or 50 years, and continue to see huge gains in productivity and global competitiveness.

It's not a completely crazy idea, but it's not the sure bet you make it out to be, either.

Regardless, it is surely better to bet on the stock market continuing to rise, even if more slowly than in the past, than it is to bet that the startup you're working for is going to be successful.

Barring a massive crisis, catastrophe, or war, it is indeed likely that the US will continue to be a growth stock for many decades to come.

It's a much safer bet than many other things.

Invest outside the US. India & China at-least will be growing 7%+ Y/Y for a quite a while. Add the rest of asia, S america, and africa, and there are tons of great investment opportunity. Heck currently euro zone bonds are making a killing.

Technically, putting your money in an index fund makes you dependent on future market performance.