If I had to guess, the idea is that Classic Sonic is supposed to be a big deal for celebrations and anniversaries. He's not really supposed to be Another Ongoing Sonic, he's the fine china you bust out for a special occasion.
Sonic Mania is a big deal, Sonic Mania 2, 3 and 4 within the spam of a decade, probably less. The problem there, though, is that Sonic Mania ended up being the game Sonic boomers were extremely insistent needed to exist and then they were proven completely correct; a 2D Sonic with just the good old characters would be the most beloved Sonic game of the last decade, and the last most beloved Sonic game also had copious Classic Sonic.
If Zelda started making 2D entries again it wouldn't get the same reception but that's because the flagship 3D entries aren't dealing with the same reception as 3D Sonic.
When Mania came out, I thought to myself, "You know, I don't need this all the time, I'd like to see the Mania devs do their own thing and not just make Sonic games. But, like, every five years or so? I'd be cool with that."
And now we're nearing that five year mark, and
nothing. And yeah, that's kinda disappointing.
But you're absolutely right in that the wider state of the franchise is to blame more than anything. If we were consistently getting good 3D entries (or solid modern style 2D entries like in the 2000s), I think the demand for more Mania wouldn't be as heavy. But Mania drops and it's the best thing this franchise has gotten in a long time when 3D Sonic was going into
another slump, and then SEGA does nothing to follow up with it games wise, and yeah, of
course people are gonna be disappointed. We want good games. Mania was a good game. Lost World was an experiment that didn't quite work out. The Boom games were average at best and mismanaged disasters at worst. Runners on mobile is dead and unplayable. Forces was mediocre. Team Sonic Racing was considered a step down from Transformed. Even the remaster of Colors had its issues, whether or not one thinks that the non-Switch issues were exaggerated or not.
It's really cool that the multimedia surrounding Sonic has been well received. The recent animations for each new release have been great. People like the IDW comics (still gotta read them) and it's great to see Ian Flynn finally involved with Sonic Frontiers. Heck, the movie that could have been a disaster is warmly loved by the fans - and I own a copy waiting to be watched. But Sonic is also a video game franchise. And to keep the analogy up, if all your other dinnerware has chips and cracks and you're not going to fix them, then either get more dinnerware or keep breaking out the fine china.
I am truly hoping that Frontiers is a good game. Classic, Modern, something completely new, it doesn't matter the direction the franchise goes so long as the games are good.