It is all fine and well to say 'anything is possible' but the fact is that people make assumptions, and assumptions based upon science are the best that we've got.
And 'science', doesn't concern itself with anything, it is just a method. People can apply the scientific method to the supernatural if they want to. The lack of evidence(due to the subject being fictional) makes analysis difficult, but that applies to any form of assessment, from faith to throwing bones. With no feedback you can't get any results. But science can actually make a pretty good go at the supernatural. You can take known values, such as people, and assess the implications of an unknown value, such as heaven, on the known value. For example, if heaven is available to everyone, and being in heaven makes you happy, then it must be possible for anyone to be permanently happy. Unfortunately, some people's nature causes them to be unhappy at times regardless of external influences. So they cannot be permanently in heaven, unless they are modified, at which point they will no longer be themselves so it still stands that they couldn't get into heaven. So we can prove, scientifically, that heaven is either exclusive, malevolent, or imperfect...