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You say that but its not like the Magic Story-line didn't involve killing characters during major conflicts to raise stakes. Heck I argue it was more Game of Thrones in the past after all even Urza died. So I think blaming it on Game of Thrones, is disingenuous to the extreme. I mean the end game against Yawgmoth resulted in all but 2 of the Nine Titans dying and most of the Weatherlight crew as well. Sealing the Time Rifts? Again more dead characters. More recently Venser Dead. Elspeth Dead. So MtG never had a problem killing major characters in the past but now the Gatewatch has unlimited plot armor.
The Gatewatch they go up against the Eldrazi and Bolas and lose not a single member. I can make a case for Bolas not killing most of the Gatewatch. Vess is obvious. Jace could be played on Ravnica since he is the guildpact. Nissa has a unique ability with her leyline stuff. After that though the case for keeping Gideon and especially Chandra around seems rather nonexistant to me. Gideon maybe cause Bolas proved his ability is useless and you want an idiot steering the enemy team as the leader. But what reason does Bolas have for keeping Chandra around?
That's one thing a lot of people hate, that winning over the Eldrazi was an "easy" victory, they were built up so much and so well in the first Zendikar block only to be turned into another generic villain to be killed in the BfZ block. And Hour of Devastation was barely even a setback, I think a lot of people were expecting half of them to die and the other half to be turned into Bolas thralls or at least one of them suffering from something that couldn't be cursed in one block.
They were built up so much and so well because you never actually saw them. It’s the Boba Fett effect, the moment you have to explain them they stop being cool. So the choice was either to literally never deal with them cause they’re too OP, or to make them more manageable. We still don’t know if there’s more to the story there, as Ugin flat out states.
Did you know that people don’t have to die for there to be stakes? There’s a really toxic attitude out there from grimdark fiction like Game of Thrones that is easily proven untrue by decades of genre literature never requiring it.