The Collison for Mahinmi trade is not as terrible as it might seem, actually. And it could still pan out (the idea of trading PG talent for big talent was a solid one). Granger being out just makes the trade look a lot worse than it actually was because we traded a gifted offensively player for someone who we don't really expect to do many things but fill in for Roy, which he has done admirably and will probably continue to improve on.
However, that entire deal was premised on the idea that (a) Augustin was a serviceable back up and (b) Green was as gifted offensively as Collison. Neither have panned out. If Augustin and Green had been as good as advertised, we'd be doing a lot better right now.
More fundamentally, this team is based on the idea that everyone has a role and is a particular kind of cog in the machinery. Danny is at the center of all of that. When you have a team of talented role players with different skills, you need the center cog even more than a team built on just a few big cogs might. That is, Dwyane Wade might be better than Danny but the role of every other player on that team is not predicated on his play. I think our team realized that we had this vulnerability in the offseason and tried to address it by getting Mahinmi (fill in cog for Roy) and Green (fill in cog for Danny). Unfortunately, Green is just not a talented scorer. And worse, he represents a huge drop off on defense as well.