I think you still have a chance of getting into your dream school. But, at the same time, I think it is reasonable to accept another offer. One of the professors I know advised me not to be too picky about PhD programs; meaning that at this stage you are a mature adult who can create his/her own learning environment and take everything the program has to offer. So why would you think you'll be unhappy at another program? I think it's a matter of personal attitude... And what stops you from doing research together with this POI even if you are not at his/her program? You can meet at conferences and also it won't do any harm to menntion that you are ready to collaborate on their research in future, if there is a chance...