"How does the QPP account for the stocks becoming less risky over time? "

I thought your point here was that large firms often move lower on the risk/return profile as they mature--and QPP accounts for that via its statistical projections.  QPP uses projected volatility on a stock to drive the projected return and QPP's volatility projections benchmark well with implied volatility (I have several articles on this).  QPP is purely statistical and is driven by market data, like most Monte Carlo models.  Implied volatility captures the market's assessment of risk over time.

QPP's standard output is for projected arithmetic return and standard deviation  rather than CAGR but CAGR is easily computed from these two outputs...

 Geoff