There are legitimate reasons for smurfs to exist, such as helping show new players the game without them getting thrown into a match with high MMR people, or for people who might need an escape from their main account (e.g. popular people who don't want to deal with people at that time).
Actively boosting people is a problem, but riot does potentially action that. It is delicate there though, since they do want people to play with their friends and friends are rarely all equal skill levels, so sometimes it's unclear whether it's active no-no boosting or just sortof 'accidental' boosting by virtue of playing with higher ranked friends. This situation is probably something that could stand to be looked at closer, particularly with dynamic queue being how it is.
The other thing with smurfs is their mmr's rapidly go nuts, so even if their level/rank is low, they'll still end up in 'fair' matches on average. I've faced off against diamonds on a sub-30 account that went on a big winning streak (bearing in mind I'm 'only' plat), so it quite possible for the smurfs mmr to overshoot their 'true' mmr and end up actually being a hindrance to the team rather than a free win ticket. Similarly smurfs frequently end up getting matched versus smurfs, so even if the game ends up snowballing one way or another, that's sometimes just the nature of the game currently and not because of the smurf aspect (as both sides had a smurf so they sortof cancelled each other out).
Also, I disagree that playing against smurfs doesn't make you a better player. It isn't fun to get wrecked I agree, but I feel like you really need someone to punish your mistakes in order to really force you to learn to correct them, and smurfs are good for that. Besides that, I feel like a huge number of players are utterly terrible at playing from behind, and anyone who wants to advance should really acquire that skill. There's countless games that were completely winnable that people lose because they fell behind a couple kills, said "gg" and afk'd at fountain until 20, and then blame smurfs as the reason they're not climbing, and that's quite painful to watch.
Lastly, there's lots of 'smurfs' that aren't, including those who say they are. For some odd reason some people seem to get an ego boost from saying they're a <insert high rank here> smurf whenever they have a good game even if they really are only silver/gold or whatever, and by saying they're a smurf from whatever elo they can feel like they really do belong at that high elo, even if in actual practice they can't reach it.