Based on overall win percentage, Dwane Casey is the most successful Raptors coach in team history. Though it feels strange to say it, a look at the record books will show a team that has steadily improved under his stewardship. There are other factors to consider here of course; the improvement of the existing roster, the collapse of the the Atlantic Division and Eastern Conference, the trades and signings made by General Manager Masai Ujiri. Still, the numbers will always show 48- and 49-win seasons, both franchise-bests, under the watch of Casey.
Is Casey the best coach in the NBA? No, obviously not. Is he the worst? Again, it is safe to say, no. In the current coaching ranks of the NBA, Casey remains a solid if unspectacular bench boss. While it's impossible to speculate what goes on behind closed doors, he at least appears to be principled in his tactics, personable with his players, and professional with his bosses and the media. (This counts for something, even that "friendly with the media" bit.) While there are certainly strategic and tactical decisions of his to question (as I'm sure you'll tell me in the Comments), it's fair to say Casey has gone about as far as he can with this particular group of players.
It's easiest of course to blame the coach. It's why they get fired all the time in the NBA, especially after a team is thought to have underachieved. The expectation for the Raptors this year was a second round appearance, minimum. They didn't make it, and flamed out in such a disastrous fashion that, as even Casey noted, people would want blood. And while a different coach could line the team up differently, or attempt to change the team's overall defensive and offensive strategy, he won't be able to suddenly make Jonas Valanciunas wiser or faster, Terrence Ross stronger and more assertive, or Greivis Vasquez an even passable defender. Even if he's lost the ears of some of the players - and many assume that Kyle Lowry's subtle non-support is evidence of this - there's no tangible reason to believe Casey, or really any reasonably effective coach, couldn't get a similar season (45+ wins, first round out) again from this group of players. That's a solid if unspectacular outcome from a solid if unspectacular coach. It also appears to be the ceiling for this flawed Raptors team. The wrinkle here is that come the 2015-16 season, the Raptors will be markedly different.
There's a scene in the film Traffic where James Brolin's General character explains to Michael Douglas' drug czar how leaders are ultimately deposed. (Thank God for Youtube here.) With the Raptors, the leader in question is not actually Casey, but rather Ujiri. He'll be the one to decide the team's next step, and he understands three things: first, he knows he has a solid coach in Casey; second, he knows this roster needs to improve; and third, he knows that if it does not improve, he'll eventually lose his job. In the Traffic movie clip, the General tells the czar an anecdote about Nikita Khrushchev handing two letters to his successor. Khrushchev tells the successor to open the first letter when he gets into his first jam, and the second letter for his second. The first letter's advice is to blame everything on his predecessor, and the second letter provides the anecdote's punchline: it says to sit down and write two letters.
In effect, Ujiri is applying a version of this anecdote to buy time for his team rebuilding efforts; he's delaying the opening of that first letter. Casey and the entire starting line-up of the Raptors were all acquired by his predecessor. The team's bench was not, but many of them are now free agents or signed (by Ujiri) to very team-favourable deals. With the coming season, Ujiri will be tasked with fixing a team that, again we can acknowledge, has reached its competitive peak. And, by keeping Casey on as coach, he's given himself one last out before having to finally open that first metaphorical letter. If things go awry next season, even with whatever personnel moves Ujiri makes, he'll have the perfect excuse: it was the coach's fault all along.
You really have to respect Ujiri's plan here, and the role Casey has within it. It's one thing to make a team competitive, as they have, and quite another to make it a championship-calibre team. Both men understand this, just as they both understand that once a team's true title window opens, you have a limited amount of time to work with before it closes (see, for example, the desperation in late 2000s-era Cleveland, OKC right now, or Casey's own 2011 Dallas Mavericks run as an assistant coach).
Casey is the most successful coach in Raptors' history, but he is clearly not the one to take this franchise to a title. He's the man for right now, the one who'll do the job upfront while Ujiri assembles a team in the background ready for the next step. It's not time to fret just yet. When you think on it hard enough, you can already picture Casey sitting down to write two letters of his own.