On the contrary, it is very much possible to become rich by working 10 to 5 if you plan it right! Having extra income source is definitely a cherry on the top of the cake. But when someone says, we can't become rich by working from 10 to 5, I only see a pessimistic person crying foul over his own fate!
In the project management field, we often make a statement - "Plan your work, work your plan and your plan will work". This statement is more true than you can possibly even think! A perfect and realistic plan always works! Yes, there will always be hiccups but the end goal will always be met if we stays on track! Start investing at early age is one of the key to become rich at right age! It is very much achievable from a 10 to 5 job!
The OP is not against working from 10-5. But, the point it working from 10-5 without other sources of income would never make anyone rich. From your statement plan your work, work your plan and your plans will work this is more or less for entrepreneurs or directed at those with a specific goal, this is not for those who choose to work for someone all their lives without plans for themselves.
Like you have mentioned of which i agree, early age investment is good but having multiple streams of income is much better because this would even help increase the investments.