Well, if you really want to take the 'canon' aspect of Uru to its ultimate conclusion.....
The idea is, Uru (and presumably Myst 5, too) is meant to be 'real life' and the other Myst games 1-4 were designed by Cyan, and based on Catherine's notes, etc, of things that happened more than a century ago... as 'historical fiction'.
Myst, therefore, is something akin to that old 'Titanic' Cd-rom game-- some events are real, some characters are based on real people--- but many events and people are 'made up' for the sake of the game.
Taken to its most extreme.. you could assume:
Myst: The books in the Myst library were burned, someone was stealing from the Ages; so Atrus wrote the 'prison Ages' of Spire and Haven to capture whoever was responsible--- and his sons became ensnared. End of story! the events of Myst, the puzzles there, etc, never actually took place. Atrus was never trapped in K'veer, and nobody ever tried to rescue either brother by giving them pages.. that's just a story written for the game.
Riven: Catherine simply got 'stuck' in Riven without a linking book.. Atrus realized he'd have to go looking for her there, when she'd been missing several days.
Atrus headed into Riven, bringing two books---- one a linking book back to Myst, and one a descriptive book for a stable Age where Gehn could live. Atrus simply arrived in Riven, approached Gehn and offered him an exchange:
"Release Catherine to me and I'll give you this Age. It is stable, prosperous and beautiful. We will go away to Myst, you can use this book-- Riven will collapse, destroying this book and closing the link to your new home... and our paths will never cross again!" And Gehn-- unable to write himself a safe, stable Age to escape from Riven -- went for it.
Myst III--- Might actually have been based on an extraordinarily short story where Saavedro appeared in Atrus' study to confront Atrus about the loss of his world.... and Atrus just directed him to a linking book that let Saavedro return home. No drama, no puzzles.
Myst 4.. Again, could have just been a simple story where both Achenar and Sirrus were released & allowed to come home; the family reunion got a little heated, and one of the brothers had to step up to keep control of the other. Heck... For all we know, both brothers may have even survived the encounter, but Cyan felt a story of loss & sacrifice was a better ending for the game.
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