5% profit of $500/year is $25/year. I'm too lazy to work out compound interest over 45 years considering that a new $25 is being added each year, however:
45 * $25 = $1125
If you invested $1125 for 45 years at an interest rate of 4%, you'd end up with $6571. (And investing $1125 over 45 years gets you more back than only adding $25 a year to the investment.)
Obviously it's possible to do better than 4%, but I would suggest that, then, however you are investing the money is "how you are making money", not "having a lifelong customer".
So, have I completely misunderstood your example, or was your maths way way off?