The two problems that you mention in your explanation are indeed problems with monopolies. However, they are not the only problems.
First of all, the lack of competition also has one other major drawback. In addition to leading to poor service and a lack of innovation, it also leads to higher prices and to lower output than customers would happen under a more competitive market structure.
Second, you should note that economists talk about how monopolies lead to a misallocation of resources. Because of the lack of competition, output is lower. This means that not as many resources are allocated to this particular sector as there "should" be. In this way, economists say that monopolies are inefficient in addition to being bad for consumers.
I see how that would apply to a natural monopoly, but not to government monopolies.