Personally, I think a Season 5 without Christopher Heyerdahl would be pointless, I'm just not that fascinated by the Helen-Will dynamic. Biggie and, to my mind, John are/were integral components of the show. Ever since they killed off Ashley (and now her replacement, Kate), the series has become more and more Amanda Tapping-centric. It's kinda like The Animals becoming Eric Burdon and The Animals (with a whole different group of "Animals") - something key is lost, even with the same lead singer, it just isn't the same act. That's why The Beatles had to break up, they couldn't become Paul & The Beatles or John & The Beatles. If it wasn't a group, a cooperative team, it wasn't what it was. Team Sanctuary started out as a real five "man" band, it wasn't Helen & the guys (a la Mick & The Stones). The team, and therefore, the show, can survive with four key members - but not three and a bunch of session musicians.
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