Let's think about this for a moment. The "original" key was simply the one that suited the artist who originally recorded a song. It isn't set in stone and there is no law that states a song must be performed in the "original" or any other key... Songs may well have been written by the composer in a completely different key to the one that they were recorded in. All the talk of singers "not being able to sing" certain songs is nonsense, I'm afraid. Peoples' voices - particularly the ranges of those voices - are the ones they were born with. Vocal training, etc may help them to be more versatile, but It is inescapable that they will be able to sing more comfortably (and therefore better) in certain keys. As musicians, it's up to us to adapt to them, so that the performance is as good as it can be. Try doing session work and insisting that you will only play songs in certain keys. You'll likely get little work (and certainly nobody will call you twice).