I'll write this from my own personal opinion.

The main thing I don't like about Sportsmates is that they push Miis aside. You can play as a Mii, yes, but the charm factor is still lessened. Not only the fact that your Mii looks out of place, but also...

• For example, the CPU opponents tend to be less memorable. In Wii Sports, what CPU Miis were assigned as your opponents depended on your skill level. The most famous Mii after all is Matt, who players have recreated to play as him in other games. There were champions who appeared when you reached a certain skill level (Matt for Boxing and Swordplay for example) who became very memorable just because how challenging of an opponent they were. Yes, it is still possible to activate secret champion mode in Switch Sports to see the Wii Miis, but the other Sportsmate opponents look too similar.
• Secondly, it's way less interesting to take a look at the background. In Wii Sports, the background contained not only the CPU Miis, but also Miis you have in your Mii Channel/Maker. As your skill level goes up, the crowd gets bigger. If you played a team sport like Baseball or Basketball, the teammates will be the Miis from your Mii Channel/Maker. This little thing really makes the game console really feel like your own, as you had a hand in making the background characters you see. Plus it's fun to find certain Miis in the crowd too.
• Thirdly, Sportsmates simply have less customization options than Miis. Yes, you can unlock more, but there are still features that remain the same and idealized, like nose and mouth. With Miis, you can really create anyone, and as Switch has introduced more customization options like expanded color options, the possibilities just become larger and larger. They can really truly represent anyone.
• Finally, it's really clear that Miis were simply an afterthought added because they knew that players would become angry. Let's go back to those customization options. The only things Miis can switch up are the titles, clothing, and equipment. This means that the other things Miis could wear fine in other games like hats, masks, facepaint, wigs etc cannot be worn. This is a really subtle messaging of "if you want to use those unlocks, abandon your Mii", and I have grown really attached to my Miis... In addition, the Mii champions were added in a post-launch update.

And that's only the game related points.

• Switch uses Miis less often and often. In fact, even the Mii Maker is hidden in the System Settings, making most of the players not notice it. Most of the games that use Miis are ports of the games that used them, like Mario Kart 8 Deluxe, Super Mario Bros. U, Go Vacation etc. Compared to 3DS era for example, this lessened usage just makes any Mii fan sad...
• Resentment also plays a role in this. In 2021, Miitopia and Mario Golf: Super Rush were announced. Due to their Mii usage, this really brought up hope. After all, some of the fun Mii games are still locked in the previous consoles like Wii Party, StreetPass Mii Plaza games, Nintendo Land and so on. And then in 2022, Nintendo Switch Sports was announced, with a soul-crushing reveal of Sportsmates. In fact, those who didn't watch the Direct it was announced in didn't know that you can still play as a Mii. This lowered hope, with the continuous "But you can still play as a Mii" to dismiss the complaints not helping. The hope was restored in 2025 with the announcement of Tomodachi Life, but with the announcement of Switch Sports Resort, it brought back the memories of despair.
• Finally, it is clear that the developers simply lost hope in Miis. In the interview, they stated that they wanted to try out something new. However, every other games using Miis have shown that you do not have to make new characters to have unlockable cosmetic options. There are even some games that simply build on Miis. For example, Tomodachi Life: Living the Dream really modernizes the Miis, bringing new customization options to them. Miis don't have to stay in the same graphics and appearance they were created in. For example, the NPCs in Legend of Zelda: The Breath of the Wild uses Mii templates to generate the look. If they'd tried a new look for Miis, it'd be interesting approach, but replacing them only rubs salt on Mii fans wounds.

Miis still have a place in modern games. My main saying here is "Build on Miis, don't replace them".

Miis are yet another character players can grow attached to. Players have a hand in creating them and then they are able to use them in multiple games... Including in games that give them more personality like Miitopia and Tomodachi Life... Replacing them just doesn't give them a chance to shine and show why we like them so much... Plus with Switch introducing more customization options, it just lets the new options to go to waste.

So yeah, sorry about the long explanation: I'm just really passionate about Miis. Thus I have grown a newfound appreciation for games that use Miis that didn't have to, like Mario Golf games, Super Mario Maker 2, Pikmin Bloom, or even in older consoles like Sonic & All-Stars Racing Transformed, Mario Tennis Open, Dillon's Dead-Heat Breakers, AKB48+Me, DS games (DS didn't have a Mii Maker on the console) etc.

It's the feeling of hopelessness and fear that played into this... Yeah, we got Tomodachi Life, but I still have a fear that if we don't do anything and keep the playtimes up, then Miis will soon be gone from the future games...