Which competitor is more responsible for the demise of Sega?

[h2ohno]

As others have said, the primary fault lies with Sega themselves.

And as others have said, the competitor most responsible is Sony.

Fear of the Playstation led Sega to make one of its big mistake in rushing the Saturn to market before it had enough games to launch and alienating many retailers. Sony was the console manufacturer that took advantage of the Saturn's high price and difficulty to program for, snatching up third parties that might have become Sega-exclusive and often having the better version of multiplats like Symphony of the Night. It was the PS2 juggernaut that killed any momentum the Dreamcast might have had despite Sega seemingly doing everything right with the Dreamcast. Maybe Sega's mistakes were so bad that even the N64 would have been able to completely trounce the Saturn on its own, but that's a counterfactual which there's no way to prove.