I'm not sure that continuous growth can be reliably predicted over a 30 year span. There is reason to believe that we may need to move away from economic systems dependent on continuous growth. (edit: though I highly suspect this is more than 30 years away.)

Buying a range of stocks and holding them for the long term may reduce your risks, but that doesn't mean it isn't speculation.

Speculation just means that you are buying something with the expectation that you will sell it for a higher value. When your assets don't produce direct value (such as dividends, rental income, functional utility, etc) then investment in those assets is speculation, regardless of the risk level or time frame.

Just because the US stock market has worked out in the past century doesn't mean it will continue to reap those types of gains. I you would have invested in the Nikkei 30 years ago, you would still be waiting.

The very act of investing is the act of taking on risk. It's not free money, you are being rewarded for taking on the risk.

Every disclaimer you will see in the trading world has the disclaimer "Past performance is not an indication of future performance".

Everything in trading is probabilities, which is a way of thinking humans mightily struggle with.

A good day to day example is when a weather forecast calls for an 80% chance for rain on the weekend, so you cancel your camping trip. It ends up not raining, and you curse the meteorologist for being wrong.

They were not wrong - yet most people say they were, showing they are unable to think in a probabilistic way.