Maybe in John Stuart Mill's time, but like anything in a free market, profit tends to zero (it generally doesn't hit zero, but gets closer over time). Those margins are calculated as part of time preference (see https://en.wikipedia.org/wiki/Time_preference). That's why you see some land owners purchase land and lose money while they wait for it to appreciate. Time is factored into the calculation.
> I wish more libertarians understood the Matthew Effect as well as they did supply and demand.
The Matthew Effect doesn't take into account the fact that many individuals get richer by requesting and receiving special legal privileges from state actors. Classical liberal thought rejects this i.e. equality before the law. That's why many libertarians are against the state's power to regulate, not because you can't have good regulation (the free market is far from perfect after all), rather because the power to regulate is the power to grant special favors. Politicians, like their private counterparts, are susceptible to self-interest. They need money to fund their campaigns. A consistent application of economic principles is necessary (see https://www.youtube.com/watch?v=JAbDrP7whqw).
Edit: Formatting