This isn't a 3rd for nothing. It's a 3rd for whatever they use that 3M x 2Y of cap space on.
On paper, this trade looks bad.
When looked at with all the other moves and the intended goal of the offseason, it's understandable. They are trying to shake up as much of a failed roster as possible and bring in veterans to change the room. The amount of roster changeover they are trying to do this offseason is very difficult.
If someone wants to argue that they think Joseph is closer to the player he showed at the start of the season and they should have kept him, that's fair. I don't think the argument that it shouldn't have cost a 3rd to move him. That's the market. They will get something in return via using that cap space, which they have already done via adding multiple free agents yesterday.