My point is that (and as you also said) investing is something for upper-income people. However, once median-income people see that all their money are in stock

My point is that (and as you also said) investing is something for upper-income people. However, once median-income people see that all their money are in stocks (even if it is 5k), it may change their attitude. This would be a light form of what people call HODL in crypto. The difference, of course, is that in crypto people expect at least 10x in 10 years while in stocks one should expect not money than 2x. However, if applied broadly, one might expect that even median-income stocks-HODLers will tend to spend less on unnecessary things.