But then again, they also don't have to worry about competition undercutting them. So it's more likely that they'll stop exploiting their employees at some point, if only because the CEO has conscience. But add in some fierce competition, and silly things like conscience have to go out of the window.
So far as I know there is absolutely no evidence that this has ever happened. The evidence is all on the opposite side - monopolies become monsters to their workers, to their communities, to their political systems, and to their customers.
The only people they don't become monsters to are their owners and shareholders.
The most beneficial monopolies are Fordist, because they realise that you need to pay workers enough to grease the wheels of consumption. But the motivation there is still exploitative - it's just pragmatically exploitative, rather than ideologically exploitative.
Private investors still want profit, but hopefully they'd be more interested (because of their larger stakes on average) in less rapacious approaches.
Although, now that I think about it, the key metric here is probably growth. As long as a company is growing, if growth >> margin to be reclaimed by being an ass, growth wins. But when growth =< that margin... there's always going to be pressure (aka Google).