I was there myself in low ranks and took detailed notes about anyone who didnt belong there. So yeah i dont think i dont underestimate issue. Its same as with cheaters, people in low ranks are constantly complaining about it and like 90% posted replay codes here on forums dont have cheater.
Well not always, i am pretty competitive person. Sometimes i chill on alt but i try hard to win last few weeks on my new main account.
Smurfing is problem because of how placement works. Blizz could seriously lower number of alts by raising Enter level to comp. Even +10 levels would do the job.