You know, I remember a castlevania game
doing something similar
Except that game was fun.
It sucked there too.
The only ones that flopped with Mario on them were Wii U & Virtual Boy (Mario Clash wasn't really a mainline Mario entry though). One could argue GameCube was a flop, but it sold better than Dreamcast and about equal to Xbox, so it's less that it was a flop per se, and more that PS2 dominated that generation as a whole.
N64 and Wii U both launched with a Mario game and flopped.
N64 did not flop. Underperforming =/= flopping. N64 sold less than SNES, but SNES sold less than NES and that didn't mean it was a flop.
Mario by itself hasn't sold a system since the NES.
Mario sold the SNES and yes, N64 too. You're just objectively wrong. SM64 outsold anything on PS1. The N64 itself may not have been as popular as PS1 as a system, but Mario was more popular than anything until Pokemon.
Mario Kart might not be a smart launch title.
I think it's a buffoonish launch title. The straightforward logic that "MK is popular so it'll be a good launch title" is true on the surface level and it'll probably sell well, but they're kneecapping the potential. MK8 still sells surprisingly well and got DLC not even 2 years ago, it's relatively fresh. MKW would've been a better year 2 release, maybe a big Christmas game.
Plus, not only are they killing an evergreen title's sales
and kneecapping their newest entry, they're also risking less interest in the console too. I'm not the only one who doesn't care too much about MKW, Mario would've made a much better launch game. After the diehards buy up the launch wave of consoles, unless they have some great games coming soon, interest is gonna dry up fast.
Making more sequels and nothing new leads to declining sales like we saw with the SNES.
That's not why it sold less, it's because the idea of upgrading hardware on a generational basis was still a relatively novel concept to people in the late 80's/early 90's. Why buy Timmy a new Nintendo, what's so "Super" about it? The old one plays Mario just fine, sounds like a marketing tactic to just sell a new thing. There's old news segments with parents talking like that.
At least with the 5th generation came an unmistakabe newness in every way, including the consoles' names, and aside from Nintendo (to their detriment), even a new media format.
Mario is 40 years old and is not going to excite people like it did in the 80s.
Almost nothing will excite people like Mario did in the 80's. It was a smaller hobby at the time and yet Mario still sold like crazy, so proportionally speaking it had a bigger impact back then than almost any game since due to how revolutionary it was.
All Mario needs is to not go the NSMB route of samey stagnation, they don't need to reinvent the wheel every generation. Since Odyssey sold & reviewed well I think it's safe to say they struck a good balance.
Trying to replace Mario with something else wasn't going to save those consoles.
Yeah, it's not like Virtual Boy would've been poised for success if only it had great games. N64 failed because of a perfect storm of Nintendo's own faults and stiff competition, not because Mario didn't excite people (it absolutely did). Wii U failed for similar reasons, but with the additional problems of bad marketing and the mistake of trying to court a fickle userbase of casuals. If anything, Mario salved N64 & Wii U to the extent possible (SM64 & MK8 were their best sellers, respectively).