Kobe came back playing after 8 months. That was too soon, but the whole warrior mentality is part of what people liked about Kobe. Players are known to push themselves to return playing as quickly as possible from long term injuries, even if it isn't in their best interests in the short term. I've seen this happen in multiple sports and a lot of the time it takes the athlete an additional 6-12 months to return to normal from when they first return back.
Now youre only looking at it for the big contract players on long term deals, what about the other players? They aren't afforded the same luxury to take be able to take 18months off to fully recover. Also in Wall and KD's cases they had new contracts to sit back on. I don't think Wall's contract had kicked in yet and because KD was a coup for the Nets and at the start of his new contract on his new team he had more leeway. Not everyone is in that position, look at poor Boogie Cousins, he could of done with another 6 months of recover, but he needed a contract, so he had to rush back.