The Sportsmates are mostly an improvement over the Miis
for this game specifically. I've said this before in a different thread but I don't think the level of customization (clothes, not physical features) we see in Nintendo Switch Sports would fit the Miis and I guess Nintendo agreed with me.
The Miis in Wii Sports worked because of their simplicity. Adding the clothes and customization options from Miitopia would be kinda missing the point. And since this seems to have a rather large focus on online play more customization options are a must.
The more realistic body proportions also work better for the new disciplines like football or volleyball. I always thought basketball in Resorts felt extremely off with the Miis due to their stilted movement. The Sportsmates don't really have this issue because they move more naturally:
The most important thing, however, is that they're seemingly adding better hair styles for non-white people:
My only concern right now is that the facial features seem to be very similar to each other. The main character art shows a variety of different eyes, eyebrows and even facial hair but the noses and the headshapes are the same from what I can tell.
What's not immediately visible in the above picture is the orange Sportsmate having wrinkles. Hopefully there's more that you can tweak.
I guess that Nintendo decided at some point that there were too many drawbacks to using the Miis over something new and they probably didn't want it to be too close to Wii Sports Club either.
People can like this or not but I don't think it suggests anything about the future of the Miis. The next Tomodachi Life game will most likely have them again in all their quirkiness.