There's a well-known debate about how Sonic's move into 3D went with one side claiming they did, and it wasn't until reviews by YouTubers that fans turned on it and the other side saying it was a rough transition from the start with the public sentiment gradually going against Sonic resulting from that. However, there's' another angle which I think reflects a more nuanced circumstance that those early 6th gen releases found themselves in. In my estimation I think those 3D Sonic titles were flawed but solid enough games that happened to be on the wrong side of a major zeitgeist shift happening roughly 2000-2002.
Geek Critique has done great video that really went into detail about the growing pains of the early aughts, but long story short, console gaming underwent as seismic shift with that 6th generation. The struggles of Nintendo with the GameCube, the greater dominance of Sony with the PS2, the departure of Sega from hardware after the Dreamcast's disappointing sales, the emergence of Microsoft with the Xbox; it all was indicative of the end of a particular era that had been going since the days of the Famicom/NES. Stuff like Halo, GTA III, & Metal Gear Solid 2 all served to challenge the idea of what console gaming was and were the titles people were pointing to as a sign of the hobby maturing.
Why this is at all relevant to Sonic? Because I do believe there was a bit of a change in both people's expectations and allowances for certain types of games that they had previously been fairly warm to. I think the issues with the likes of SA1 & SA2 were ones that a lot of players were willing to forgive once upon a time, but once there were so many disruptions to conventional gaming and how it was entering its adolescence, what were flaws that the broader playerbase were willing to look past suddenly became major holes in the proverbial armor of Sonic games. An enjoyable speedy platformer with some weak sections and hit and miss story segments wasn't going to cut it now to a lot of gamers that were seeing new forms of storytelling and seemingly redefined gameplay in other titles.
Platformers in general got hit with a lot of understated dismissal in this period and in some ways the genre didn't really broadly recover from it until the retro 2D variations got big and the nostalgia for 3D ones started kicking in around sometime late in the 7th gen. Mario escaped this to a degree, yet even that was largely because the series historical significance gave it token respect and that mainline titles went into a major hibernation (with only one new big Mario release coming out between July 1996 and April 2006). That the positive reception to that one game (Sunshine) was fairly restrained compared to what came before and after is telling in its own way; even Mario wasn't really wowing people as much anymore.
Essentially those early 3D Sonic titles may not have had the same consistency as the various 16 bit entries, they still had clear value as technologically impressive hardware showcases and made for pretty fun experiences. However, the problems that were present (and make no mistake they were there) got put under a much harsher microscope from a gaming world that was seeing so many conventions challenged and so many old styles of genres suddenly coming off as outdated. In that sense it's not really a surprise that younger fans largely unaware or uncaring about this proverbial conversation were the ones that would later elevate the Adventure titles and be so baffled by how quickly opinions soured on it. We're so far removed that context that it is tricky to really get it unless you were there, or you dig deep into the discussion/reviews going on at the time.