Democrats should, of course, actually be celebrating because the worst thing that can happen to a political party is to win an election. It puts them into a position in which they will demonstrate to voters and to the world that they are woefully incapable of governing.
We have been through this many times. Democrats win control and accomplish essentially nothing that they promised to do. Having no accomplishments on which to campaign, they use the "we're feckless but the other party is evil" approach to stay in power, only to have the voters say that Republicans weren't all that evil last time and getting something done is better than getting nothing done and throw them out.
Republicans get control and get things done, but they get all the wrong things done. They try to convince voters that those things are actually good ideas, but voters decide that maybe doing nothing is better than doing stupid things and put the Democrats back in office. Wait five minutes, rinse and repeat.