Stopped competing? What does that mean? To me that's akin to one of my least favorite sports platitudes, "he/they wanted it more" which often takes the place of a more nuanced or comprehensive answer for an end result.
The Kings have definitely played a much worse brand of basketball over the last few weeks. How much of that is due to a decrease in effort? I don't know, but assigning it as the primarily or even only reason seems like specious logic to me. Essentially your argument is that a team who, after 24 games, was told their coach was fired because the team wasn't playing the right way should produce exactly the same results despite having to learn a new system during the season while playing for a coach that no one expects to be back next year?
D'Alessandro said that Corbin shouldn't be judged yet because Cousins is only "60%". And yet, you're having your 60% star center (who is 6'11" and 280 lbs by the way) play more minutes per game at a significantly higher pace than at the beginning of the season when he was ostensibly completely healthy and you think that will have better results? And if the players are seeing that the new "style" is costing them wins and the style the coach they liked was fired for playing was allowing them to compete how sold on the system can they be? Even if you want to compete, subconsciously it makes it very hard to give full effort if you don't agree with the direction you're being given and/or don't have any trust in the future of the organization you're a part of.
Just as an aside, not every player in the NBA loves playing basketball. I know it seems that it should be the case, but from a few conversations I've had over the years I can tell that it isn't.