If there was a man named Call of Duty, I'd name him just as resilient. 
But in all seriousness, as someone who kinda hates Sonic, it is pretty impressive that he's still alive after catastrophes such as Shadow the Hedgehog, Sonic 06, and Sonic Boom Rise of Lyric.
I guess it's cause games like the original triolgy on Genesis are just so iconic, Adventure 1 and 2 were pretty impressive at the time, and somewhat more recent games like Colors and generations got pretty good reception, with Generations being my fav Sonic game of all time, so whenever Sonic games are good, they stick out.
Also, the Sonic fanbase is easily one of the more crazier fanbase out there, and if the series were ever to be stopped, no doubt that there would be bloodshed throughout the entire internet.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."
