there's quite a bit of context when it comes into understand why ppl specifically think 2006~onward was the darkest age of sonic
with SADX, SA2B, and mega collection releasing on the gamecube, there was a newer generation of fans experiencing sonic for the first time, and that audience grew larger when sonic heroes released due to it being multi-platform
but you're right, when shadow the hedgehog released in 2005 it was so wildly different compared to previous sonic games, and even marketed as such (with the gunplay, and more "mature" dialogue) that a lot of ppl were really divided on how they felt abt sonic the hedgehog as a series
sonic '06 had dangerously high expectations to hold up to, and it (obviously) didn't live up to any of that, and even after 15 yrs after the game has released we're still feeling the consequences of that. not many ppl can talk abt the time sonic x-treme was cancelled, or the dreamcast being discontinued, but
everyone can talk abt the time "sonic kissed a princess". I think it also didn't help that the games following '06 were black knight, unleashed, and secret rings which further soured ppls interest in "sonic and his stupid friend" which is unfortunate bcuz that is where most ppls experience w/ the sonic franchise is stuck at despite it having so much more to offer
as for the brightest, I'll agree with you, the highlight of the series was absolutely at the beginning, it was real
sonic mania!
but I think sonic has had other bright times as well, from 2001~2003 with SADX, SA2B, and mega collection on gamecube, and then sonic x airing on TV it felt like a second wave almost of sonic hype, but I could also just be horribly nostalgic abt such a time