SEGA is being devalued piecemeal. Tencent took Nagoshi and Nintendo has over the years taken in quite a bit of Sonic Team at their Tokyo studio.
Not to mention they bypassed SEGA when dealing with Platinum Games. Also, Nintendo bowing out of the Tokyo Olympics because of all the imperial nationalism also punched a big hole in the planned Mario / Sonic Olympics cross-promotion campaigns.
In the meantime SEGA Sammy has restructured so that an acquisition of just their gaming business is easy and dropped the arcade business and other ventures that could scare potential buyers.
[citation needed]
https://metro.co.uk/2021/08/30/sega-may-lose-yakuza-monkey-ball-creator-to-chinese-company-netease-15177021/[citation needed]
Tetsuya Kobayashi (Production Manager Sonic Unleashed - Mario Kart 8: Deluxe Course Design)
Makoto Yonezu (lead field artist on Sonic Unleashed - Planet designer on Galaxy 2 - lead landscape artist on Zelda BOTW)
Daisuke Amakawa (field artist on Sonic Unleashed - Character modeling for Zelda BOTW)
Hideaki Hakozaki (field artist on Sonic Unleashed - Producer Xenoblade Chronicles X)
Keihan Fujii (field artist Sonic Unleashed - Landscape Modeling Zelda BOTW)
Ryoichi Yamagishi (field artist Sonic Generations - Course Design Mario Kart 8: Deluxe)
Masahiro Ukai (Programmer Sonic Generations - Programmer Xenoblade Chronicles 2)
and others, Square Enix and Nintendo took the biggest chunk of Sonic Team devs I believe.
[citation needed]
https://www.videogameschronicle.com/news/nintendo-reportedly-pulled-out-of-the-tokyo-2020-opening-at-the-last-minute/They pulled a number of campaigns centered around the Mario & Sonic game as well.
[citation needed]
https://gamingbolt.com/sega-restructures-gaming-and-pachinko-businesses-yakuza-creator-steps-down-as-cco[citation needed]
https://www.eurogamer.net/articles/2022-01-28-sega-quits-japanese-arcade-business-after-50-years"Bypass Sega" didn't they need their explicit approval to publish the sequels?
Pretty sneaky bypassing dozens of layers of public corporate scrutiny... just to... slight? a longtime corporate partner... 
Bypass might not be right word, but there was a different version of the deal where SEGA would've played a bigger part in production and development (and Nintendo paying them more) than just allowing Nintendo/Platinum Games to use the IP.
From what I was told in general SEGA had hoped Nintendo would expand on Mario & Sonic (and Smash Bros.) and Sonic would show up in more games as a guest character.
You can imagine how they felt when Nintendo added a Mercedes GLA to Mario Kart instead of Sonic

There's not necessarily any bad blood between modern day Nintendo and modern day SEGA it's just that Nintendo is not an easy or very supportive partner to work with.
SEGA has wanted to create a Mario & Sonic platformer for a long time and Nintendo always balked at the idea.
https://segabits.com/blog/2011/05/16/mario-sonic-platformer-never-happen-do-to-internal-disagreement/Do not forget all the times studios that worked closely with Nintendo went (nearly) belly up because they either left them in the dark or suddenly changed direction.
Nintendo has the luxury of sitting on games for a decade if they feel like it but most developers don't. SEGA is sort of in the middle that they can afford missing an exclusive deal or cancellation or two but at some point it's going to hurt.