Maybe revolutionary was an exaggeration, but I would argue that series that used to be creative but got stale for decades would constitute regression, yes. Like Forgotten Land is indeed ambitious specifically in the context of modern kirby, but compared to other 3d platformers is nowhere as impressive as what Super Star was compared to other 16 bit platformers. I feel similarly about the others.
I'd still argue that Return to Dreamland was the breath of fresh air the franchise needed after a full decade of handheld only spin-offs, remakes and games not developed by HAL themselves.
I still like some of them like Canvas Curse for bringing the Soul Boss trend to the series and Superstar Ultra for having many more side games added to it but in the same way I regret that the Gamecube Kirby game got cancelled.
Kirby is primarily a handheld franchise (it even started on the Gameboy after all) so I understand why they focused mostly on it (that's also why the 3DS had more Kirby games overall).
On a side note I think that Planet Robobot is up there with Superstar as one of the best entries in the franchise, both pushing it forward while also keeping its roots.
The playerbase of the Kirby franchise still agrees that RTDL is one of the best entries in the entire saga as well (even if I've discovered it quite late I do agree despite the quality of life the subsequent games have added).
I wouldn't say it got stale for decades, only one for the 00's.
I would argue that all of the mainline kirby games are pretty basic platformers at the end of the day and forgotten land doesn't really change that all that much. It's still a pretty decent platformer. Far from being "regressed" at least. I had fun with it. Same with super mario wonder.
Nintendo has a pretty good track record with their games overall imho.
Wasn't it seen as one of the best selling game and almost as good as RTDL was for the side scroller series?
Wonders is what NSMB 2 should've been imo.
I almost expected 3D Land and 3D World to replace NSMB back in the 3DS/Wii U days.