With Hwy 407 tolls can only be increased only if minimum volume levels per lane are met and that has not been an issue given the strong growth in that area.
I would argue that this is one of the benefits of a privately owned highway - the highway operator will add lanes if it allows them to make more money. A government run highway with no tolling has much less incentive to add lanes since it isn't like the Deputy Minster of the MOT is going to get a big bonus because highway congestion was reduced. And you have seen the 407 add a lot of lane capacity since the highway opened. In this way there is good alignment of incentives between the highway operator and customers of the highway. I don't use the highway much since I live far south of it, but I do normally take it when I go from my home in Scarborough to the Hamilton area as it is very likely to save me time as the alternatives (QEW,403,401) often have traffic jams.