I am against moral hedonism.
If moral hedonism is true, according to the moral norm, people will not do something which will make them feel pain. Thus people will plug in to gain more experience but I think most people will not do that.
First, we can imagine that if that machine works for a long time, will we be not able to feel happy anymore? If we use the machine it will be too easy for people to get pleasure so a lot of people will not choose to work or eat, which can bring them pleasure.
Second, if that is correct, the threshold of happiness for human beings will increase tremendously. At that time, will people pursue happiness any more? The desire for people is not easy to satisfy and there will be a disaster. Thus this will not be moral and impossible.