Boosts didnt devalue the leveling experience. By the time of Wotlk, it just became too cumbersome for alts and new players, and blizzard introduced heirlooms to try and deal with the alt part. Then cataclysm is what fully devalued the leveling experience, making it an ultra-fast journey through the revamped/streamlined azeroth.
Yes, boosts will have some negative effects. They will also have some positive effects. No, they wont be the end of classic as many imply, and boosts will not go away no matter how many forum threads there are. Blizzard is not in the habit of listening when the players say “Please dont sell us something that you will make a lot of money on”.