For what it's worth, I interpreted this exchange as Dumbledore recognizing why it would be bad for someone to read Harry's mind. In other words, a competent plotter who didn't have society's interest at heart could implement Harry's ideas successfully to cause significant harm. I didn't take the exchange to show that D believed the ideas wouldn't work basically as intended with a minimum of unanticipated consequences.

In short, Lucius Malfoy shouldn't be able to read Harry's mind to gain a destabilizing amount of wealth.