Haikyuu is basically a conscious attempt by Furudate to improve on Slam Dunk's shortcomings

Eh, I don't like that mindset that Slam Dunk had "Problems" that a story written 2 decades later "Fixes". You would hope that newer manga are more streamlined than its predecessors. That's not a knock on Haikyuu as it's a great story. But all things considered, It's all about the manga artists' intent.

The early chapters focusing less on basketball and more on the delinquent aspect is likely just the era it was written in. Those type of manga were popular during the 80s, so it would make sense that Inoue was likely influenced by that and wrote it to appeal to people who were also into those stories and then slowly flesh out the basketball side of things. Plus, most of the kids on the basketball team were punks who liked getting into fights and such.

The main thing Inoue was going for with Slam Dunk was realism. He didn't want to write the typical Shonen Jump story of a main character who exemplifies courage and friendship and overcomes no matter the odds. He wanted to focus on the human element of things, so he wrote about a punk character named Sakuragi, who initially feigns interest in basketball to get the girl. But as he goes through the journey of playing the game and learning the game, his initial lie turns into truth. By the end of the story, he genuinely loves basketball and puts his body on the line to be there for his teammates as they overcome Sannoh. From what Inoue has said, it was his intent to write the ending the way he did since he was coming up with the Inter High bracket. Shohoku would overcome Sannoh...and then go on to get crushed off screen by some other team because it was realistic, and how in life things don't always work out as one would hope for them to.

I don't think every character needs to have some detailed backstory to make you like the character or even sympathize with them. Nor do they need their own arc. Most characters didn't have that and thats ok. For most of the rest of the cast that comprises the Shohoku basketball team, the few that got some extra flashbacks and such was fine. It was how the team bonds, learns to play together and all of that, that makes the reader care for them too and be invested in them.