I don't think there ever was a Sonic cycle. That basically just amounts to people being overdramatic. You could count on one hand the number of truly bad games in the series, and even among them you'll find them to have their dedicated fanbases. Sonic's had a much bigger problem with occasional mediocrity than with having much in the way of bad games.
In regards to Frontiers in specific, I don't really see what people are complaining about here. Some of what people are saying makes no sense in regards to what's been shown off (Anyone calling this Forces 2 is frankly just lying to themselves. The two games look nothing alike other than Sonic using the same character design), some of it is based on assumptions, etc. The only proper criticism I've seen is in regards to the draw distance and pop-in, as well as the bird not being animated enough. We don't know how button mashy the combat is (there's no prompts to indicate what buttons are being pressed to perform different actions), we don't know how fast paced the game will truly be (it's pretty obvious that the SEGA employee playing the game in these teasers so far is playing bad on purpose to show off the movement and combat, rather than like how a real player will play it), we don't know how empty later locations and islands will be, etc.
I feel like people are coming up with too many reasons to panic based on absurd assumptions, claims, and excuses. I'm not saying the game is going to be good, but what I am saying is that based on what we have seen so far I haven't seen anything to warrant the negative feedback I'm seeing in the fandom. It really doesn't look that bad right now, at worst it just looks like it might be a little boring but even that is strictly speculation without more information.