Personally I think the Japanese PM was jumping the gun (excuse the pun) a bit by saying the Games will go ahead. Of course, the outlook for his own country and China is already looking better with the number of cases reducing, and if only the Japanese were competing he'd be correct.
However, in Europe the virus spread is unfortunately still in its infancy, and while the number of confirmed cases (and subsequent deaths) increase day on day, the control measures are going to remain or get even tighter. So athletes will be on lockdown and will not be able to train for a considerable amount of time, and should the lockdown in Europe continue through to May, the Games will not go ahead as the competitors have not had enough preparation time.
For the Japanese PM, the decision will not lie with him but the OCC who will most likely postpone the Games.