If memory recalls, most of this forum was convinced Brooklyn would drop from being a 38 win playoff team to the worst team in the league simply because they were giving Deron Williams' minutes to Jarret Jack. I was told Jarret Jack was so incomprehensibly bad that Brooklyn would be hopeless.
So who knows... maybe swapping out Jarret Jack for Shane Larkin will not really matter.
I still think Brooklyn is a threat to dig out of the bottom 5 as long as Brook Lopez is there.
That was just one of the major reasons, not the only reason. Statistical data argued that Deron was much more effective than Jack despite the decline in his game, and losing Deron really diminished the quality at the PG position, as Who pointed out.
I think the biggest reason why most people were convinced Brooklyn would be horrible is their lack of depth. Looking at their roster at the start of the season, it was quite clear that the were ill-equipped to deal with any major injury.
And swapping Jack for Larkin will probably matter a lot. Jack was playing well for much of the season. He's a much better scorer and passer than Larkin, and offers more size at the PG spot. Larkin and Sloan = worst PG combo in the entire league.