People that like / agree with SBMM in casual modes have no understanding of how it actually affects players, and I genuinely could care less how much knowledge they have on the game.
SBMM is fine in a scenario where players are given rewards for climbing the ranks, but they aren't in regular multiplayer. You're basically shown that the better you are, the less fun you'll have.
If it were solely ping based, you'd know that the better you get, the more you'll excel versus the enemies, giving incentive to become better. Of course, that would also come with more playlists, as they can fill every lobby. -oh, and wouldn't you know it, better server connections across the board.
SBMM disincentivizes players from becoming better, but it also makes them lose understanding of how many people are actually good / bad. This exactly is what's lead to the massive cheating pandemic in multiplayer games.
After interviewing 100+ cheaters in Call of Duty, by far the most common answer I've been given as to why they choose to cheat is "every player I'm fighting cheats, so why shouldn't I?"
Probably less than 1% of players cheat. But, thanks to SBMM, people can't believe their eyes when they're shown a player that's better than them. Sure, there's times where it's spectator bugs, and things of that nature, but in most cases it's just skill.
Now we have a player base completely divided due to a matchmaking system, which has the sole goal of making money for a company that doesn't give a f**k about your experience, only the amount of time you play.
Oh, and don't get me started on how terrible Engagement Optimized Matchmaking is. Imagine the TikTok algorithm, but for multiplayer games. They find what makes you stay on and find a way to repeat that as much as possible, even if it ruins the experience for you or other players.
Regardless, my point is this:
I don't want easy players all the time. If I did, I'd hop into a private match and turn on Recruit level bots. I want the chance to experience matches where I'm benefitted for being a good player, not punished.
That means I want to fight, win, and lose against good, bad, and decent players. But if every single game is against players at a similar / better skill level, then each lobby becomes the exact same. There's nothing new, nothing different.
The only reason I play multiplayer in this age is because I have to unlock things for Battle Royale modes - which they know, and gives reason to why they don't allow you to immediately use all items in BRs.
If you can't see past the bullshit and realize that you should always be given a chance to play against every player-type, you have zero perspective. SBMM is negative on all fronts aside from making money for the companies that implement it in casual game modes.