If you look at peoples' gross paycheck income, it appears to have stagnated, but much of this is due to the rising costs of healthcare. Incomes are effectively continuing to rise, it's just that much of that increase is being funneled into benefits rather than take home pay.

Then again, it's also important to bear in mind that most common investments are made in stocks and bonds, not in workers. In this case you're investing in companies and/or government securities, not employees, so falling incomes do not necessarily imply decreased ROI.

To some extent you are right: obviously someone has to do actual work, and not everyone can just make money on investing. That said, it's not a one to one correspondance: if more people are investing, that means there's more capital available for companies to build off of, which means it's easier to get a profitable company off the ground, which leads to more economic growth.