I don't disagree about the impact of Sonic's debut, though not to the degree of Mario. Still, Sonic was the prototypical 90s video game mascot. But the discussion is more about personal enjoyment rather than industry impact. I've spent more time with Sonic 2.
Also, I don't think the argument that Sonic 2 is just Sonic 1 with DLC really sticks. It's not the same game with merely expanded content or different modes. All the levels, boss battles, and even some mechanics are totally new. Not to mention additional importance placed on collecting the chaos emeralds. Super Sonic FTW!
I would say the step from Sonic 1 to 2 is more analogous to the step from Megaman 2 to 3 (or even 4). It's essentially the same engine and graphics but new levels, bosses, and added mechanics.