A smurf is equivalent to an NBA player playing in a high school game. They are taller so they can shoot over their opponents. They make more baskets consistently, so they can score more points. And they can do this while playing “casually”. Inevitably they will cause “their team” to win, even though that teams is objectively weaker than the opponent without the help of the “smurf”.
You can say the smurf is a paying customer, but so are the other 11 players. Eventually those players are going to get frustrated and stop playing the game, which means they will be less likely to buy loot boxes or recommend the game to others.
Smurfs also make it harder to catch cheaters because their mechanical skill is so good lower rank players cannot tell mid-game. The only way to catch a cheater these days is to watch the reply and slow down the gameplay to find instant snaps, walling, or the player randomly shooting the ground between team fights.
While smurfing is bad and has been a problem, it probably isn’t as bad as people think. GM and masters players make up 1% of the population. That means at best you would get one 1 out of 10 games, if they were all doing it. More likely a “smurf” is someone who is 1 rank higher or just better. Or someone who says they are to tilt their opponents. It’s not worth the mental energy to worry about it.
Also if there are smurfs in a game, they are just as likely to be on your team than on the enemies team.
Best thing is just to watch the replays and focus on your own mistakes and try to make fewer of them.