That doesn't mean that there is no market failure. Monopolies and oligopolies often do still provide improvements, but it is no longer dictated by what is possible, but what they consider optimal to maximize their profits.
The problem with this claim is that 10 years ago there were still only two major players in the GPU space. Hell, that's been the case for almost two decades. Why did it take so long for them to decide to be greedy?
It seems like other factors offer better explanations than an alarmist claim of market failure.