I'm not sure about the exact time frame, but I think it was maybe around when he had 10 million subscribers that he started not playing Amnesia as much. So his current subscriber count would not reflect the amount of people that watched him play Amnesia at the time.
That being said, it may also be likely that new subscribers dug into his Amnesia videos , because that was one of Pewdie's most iconic series.
I do not believe that Pewdie was the person that without which would've made Amnesia sink into the ground, and no one would've ever heard of. He definitely helped, but he was not the sole man who brought Amnesia into the public eye. That was done alongside various other Youtubers at the time, the video game press, and the general fact that it's a really good horror game which is a kind that has not been seen for a really long time.