I think the NBA became a job just how rap became a job and dudes aren't doing either to become the best but just to get a paycheck and it shows in the quality of the sport.
It's always been a job, breh. And if you think players were trying any harder in the 60s-70s-80s than they are today, you haven't watched enough regular season game film.
In terms of "playing hard for 48/82", the peak of the sport was from the mid-1990s up to the mid-2010s. If anything, it was those early 2010s teams who were the absolute peak of playing hard on both ends every minute of the game (those Bulls, Heat, Celtics, Pacers, Spurs squads who did not let up). Until then, effort - especially defensive effort - had pretty much increased in every decade of the sport, cause it was an arms race - you could only play lax on defense as long as other teams were playing lax on defense, once one team ramped up the effort than the ones who didn't increase in effort got left behind.
We have seen a partial drop-off in effort since the mid-2010s, but that's due to two reasons that are directly related to the game. One, injuries became such a massive part of the sport that coaches and players just had to let up or they'd never make it to the postseason. And two, the spreading of the floor with the increased emphasis on the three-ball just made the modern sport impossible to guard all-out for 48 minutes without running yourself into the ground.
It's usually possible to explain trends with real factors rather than convenient narratives.
melo lost sponsorship deals that 8 years prior Barkley was loved for. 
off the fact u turn on the tv and its a bunch of light brite Orlando Magic Anthony Black looking nikkas with afros all over the place