Dedicated servers aren't feasible it would require them to fully rebuild the game. And even than with how destiny has done in the last 2 Dlcs Sony would never give the money or devs for it as it wouldn't be able to recover costs.
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Yet they can afford dedicated servers for Marathon. They could have been redesigning the network design for ages.
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Totally different. Marathon is a new game with a chance a high earnings. Destiny is a decade old with a multitude of problems that can't be easily fixed and a dwindling player and profit line.