It would be close and both eras have massive adv.
The old NBA had three-four year college kids to the pros. None of this one and done bullshyt where nikkas like Trae don't really understand how to win or play this game at that high of a level.
The new NBA got internationals. Them mfers changed the game. They come in at 19 more advanced mentally than those 3-4 kids of the old NBA. They just don't have the athletic ability as those then or the US kids now.
Steph wouldn't be Steph in the 90s. His skillset even in college was freakish offensively. He clearly had NBA training shooting with range. On top of that, Steph is one of the most trained players ever. He is trained as a machine. No one had that access to training like that in 1990. No one. Not even Jordan nor was he close. Steph has like five trainers. MJ was seen as magical having just one. Steph is also great movement-wise, stamina wise, hustle wise, and BBIQ wise. Green is like a QB. Klay would be impossible to guard in the old NBA. Their ball movement was magical that no team has come close to, to this day.
I don't like comparing teams from other eras because let's be clear, that 73 win GS team would sweep every NBA team from 1980-2014. That team made no sense for its era, it would have killed any era squad. No one could compete. They had the best tech. The best trainers. Coaches with the best schemes. They ran that M2M/zone split to perfection. Their screens were insane. They could all put the ball on the deck. Everyone had size. Everyone would be a nightmare for every era. It would be impossible to compete. LeBron needed luck and Green's suspension to beat them.
It would be close. Being able to have most of the NBA know how to play Basketball is a BIG DEAL which is the issue in the NBA today. That said, the talent pool is massive. Because of guys like Jordan, the training from kids starting at 5 years old for a sport is massive. MJ didn't start playing till he was 14. Bo Jackson mentioned there will never be another him or Deion because the talent pool on all sports is much too great due to advanced training.
What did you expect? This game is a game where players are making nine figures over their careers. nikkas like Kent Bazemore. The training to be in this league was going to be massive due to the money in the NBA. The money before the NBA, etc.
The game is just a lot more advanced today. That doesn't mean it better but the shyt kids coming in can do in 2020 is a lot more than kids could in 1990. It's not even close.