it was never good.
There's a distaste the broader culture develops around a property when they get the feeling that it has a fan-base bigger than what the quality of the thing warrant (see: twilight). If there's a franchise that has a massive, incredibly rabid fan-base but the entries in that franchise are all mediocre at best the go-to line is "that franchise should just get back to when it was good". And I'm here to spoil for you why Sonic doesn't get back to when it was good.
The problem is it was never good.
People aren't fans of Sonic bc the games were good. Most people who are fans of Sonic never played a Sonic game they liked, or even played one at all. They're fans bc of its marketing.
A 15 year old in 2009 wasn't posting OC bc the old games were good -- they'd never played the old games; they had never even seen an NES. They were posting OC bc before they had a PC with MS paint, their parents bought them coloring books & they spent their free time before they had a PC making Sonic OC on Sonic coloring books.
The games were never good to begin with tho. All of the best Sonic games of all time had exactly 1 good level each. Sega spent an obscene amount of time & energy marketing the franchise -- they even went so far as to collaborate with Michael Jackson in several ways (the character is partially based on him), but in terms of actually making the games they only put in the time & effort of making 1 good level for each game, made it the first level which everyone would play the most bc the game would kick you back to the main menu when you ran out of lives and then just shipped it.
TLDR; the games were never good, Sonic only had its fandom bc of the coloring books, there is no golden age of "Good Sonic Games" to go back to & if you've spent your whole life thinking or mis-remembering that there was you have been fooled by a marketing ploy.
I'm not even a sonic fan and I know this doesn't hold water. People do genuinely love these games. You can say it's based in nostalgia, or you can point out that different people think whichever one they played as a kid was the peak of the franchise, or make a case that it's not "organic" success (as if such a category even makes sense in the capitalist cultural marketplace), but it's completely wrong to say that fans of the series have never actually played the games.
This smells a lot to me like /v/'s attitude of "I don't enjoy this game, and it's literally incomprehensible that anyone has different tastes from my own, so they must all be lying to me and to themselves."
it also never came out on NES, so im guessing OP wasnt around when it was first released. probably thinks the 90s where one big vaporwave pepsi ad too