You are right. The way the system is now, people should take advantage of the available means to make money that are out there.
I honestly don't have much of an argument for people who do not avail themselves of all opportunities for growth. I would make a small caveat, granted your niece is likely the exception and not the rule, there aren't a whole lot of people with 50k jobs who have time or money left over in the day to take advantage of the market.
Regarding the super rich.
Again, sourced from a leftist leaning source, but I don't think you can argue that the statistics evaluation is off.
http://www.faireconomy.org/research/TrickleDown.html
In summation the article performs a statistical correlation between top tax rates to determine the correlation coefficient between top tax rates and various economic indicators for the period 1954 to 2004. In all cases but one, there has never been a significant statistical correlation between lower taxes and anything that would be considered "trickle down", and in that outlier it was a fairly anemic link.
1. Cutting the top tax rate does not lead to economic growth. - Correlation coefficient 0.03
2. Cutting the top tax rate does not lead to income growth. - Correlation coefficient 0.06
3. Cutting the top tax rate does not lead to wage growth. - Correlation coefficient (Didn't have for this one, however the correlation between raising taxes and wage growth was 0.34, which indicates higher taxes correlated to higher wage growth on the peons)
4. Cutting the top tax rate does not lead to job creation. - Correlation coefficient 0.11 (some indication that people do hire more when taxes are lower)