From the outside perspective of someone who's not a basketball fan, it seems to me that the main downsides to trading DeRozan are all sentimental. He's a career Raptor and unlike all the other Raptor stars before him, he chose to stay and embraced the team and the city. So in that respect, it's a shame to ship him out---doubly so for a guy that apparently has no interest in being here.
But DD isn't an innocent victim here. Sports are a results-driven business, and he's repeatedly proven that his game just doesn't translate to personal or team success in the playoffs, despite him being surrounded with an ever-improving supporting cast. Coming back once again with a team headlined by him and Lowry would have been the definition of insanity. Even with Lebron gone from the East, a DD/Lowry-led Raptors team getting to the third round, emerging from the East and/or putting up any kind of fight in the final was exceedingly unlikely.
So they trade him, one of their lesser young players and a 1st round pick that's guaranteed to be low. In return, they get to compete for one season with one of the best players in the sport on their team. In a best-case scenario, they sell him on sticking around over the course of the next 12 months, and with that all-important 1st superstar in place, maybe they finally convince another star free agent to join him here. In a worst-case scenario Leonard is a bust due to injury or a lack of desire to play here, he walks at the end of the year, and then the Raptors embark on the rebuild/reload they would've had to go through anyway, with all of their future 1st round picks & best young players still in place and more cap flexibility.
Hard to see the downside here.