I'm about to give my honest opinions about the Mario games because I don't think I've given any full thought about the franchise that I grew up with.
I love Mario. Probably my favorite franchise ever because of the variety of genres the italian plumber has been through... but I do think the franchise has had some issues especially as of recent years.
I think the best example of Nintendo just not giving much effort to the plumber because of his success is New Super Mario Bros U (and yes, I'm talking about both the Wii U version and the Switch rerelease). That game was just NSMBW just dumbed down for super casuals.
The worlds were so generic, the gameplay was the exact same as before on the Wii, the music was almost identical, and the characters weren't interesting at all.
I understand that this isn't supposed to be a challenging game, but a game accessible to all players... but if you tell me Nabbit is acceptible as a character, I will beat you to death. Nabbit completely breaks the game and is invincible to almost everything.
One reason why I despise this game so much is because there are other good 2D platformers like Shovel Knight, the Messenger, Yooka-Laylee 2, etc. and all Nintendo can do, for their most successful platformer franchise, is give us this shit? No! I won't tolerate this TWICE!!!
And it's not just this game, but there were many others that suffered from little to no effort. Mario Tennis Ultra Smash and Super Mario Party are some other good examples of Nintendo milking their franchises with nothing.
Even games like Mario Kart 8 seem generic as hell, especially with the roster. They had 7 koopalings replace Diddy Kong and Funky Kong, and there was a Pink Gold Peach, who no one asked for. The only cool characters were the DLC ones because they were NEW and not milked.
The only actual good Mario game for the past few years was Super Mario Odyssey. That game killed it with its fresh gameplay, new environments, outstanding music, etc. It's my 2nd favorite Mario game to this date (behind SMW of course).
I believe if Nintendo just gave that treatment to all the games to come up later, Mario could become "good" again. I'm not saying he's fine now, but he's definitely being given little development in his games.
Super Mario Maker 2, to be honest, leaves a lot still to be desired. While the whole experience was expanded upon, the environments aren't too interesting, most of the songs are generic as hell, and the online sucks (that though is completely different).
What's worse is that Nintendo does almost nothing to this franchise, and they still don't do jack to F-ZERO, Metroid, Kid Icarus, etc. There are many franchises Nintendo hasn't given too much care for and yet they think they're doing good with Mario.
I think the Mario franchise was once a good series that Nintendo spent a lot of time on, but arguing that the GameCube - NES eras were better than now is wrong. Standards back then were different than they are now.
Not too many good games today too really get the attention that Mario does because of how big he, like Yooka-Laylee 2, the Messenger, etc. If those games were as big as Mario, they'd get so much more attention than just enthusiasts like myself and others.
If Nintendo had to make one game a year just so that Mario could finally get good, then I'm willing to take that risk because they could also spend time working on other franchises too.