I've recently seen some complaints about the movie from fans who only watched the original anime and haven't read the manga, such as:

• Miyagi's backstory comes out of nowhere

• The humour and funny tone of the anime is missing

• It doesn't have any of the music from the anime

• It lacks the charm of the anime due to the humour and the original music not being present

Before I begin, I should mention that I'm also an anime-only fan, but I did pick up the manga from where the anime left off.

I agree with you that the 90s anime has its own charm that makes it really special. The night before I went to watch the movie a few days ago, I stayed up late flipping through my series illustration books while listening to the old anime's soundtrack. As much as I truly love the music of the old anime, I feel like it would've been an awkward fit in a modern adaptation like this. This movie needed to be its own thing (one of the reasons why Inoue named it 'The First Slam Dunk' is because he wanted it to be Slam Dunk as you've never seen it before, like you're watching it for the first time), and it really needed to have its own soundtrack to avoid hinging on the old anime.

As for the humour and funny tone of the anime that you're talking about, this movie was not without its funny moments - the jokes from the manga were there, such as:

• Sakuragi not knowing the name of the team he's playing

• The signal that Sakuragi and Miyagi used for the alley-oop at the beginning

• The silly tutorial of how rebounding works

The thing is, there's a time and place for the humour. Think back to the really important matches against Shoyo, Kainan, and Ryonan. There were moments of comedic relief, sure (and again, this Sannoh match had those moments too), but overall the tone for those matches was intensity. Moreover, the Sannoh match is supposed to be so much more intense than all of those matches. Too much humour takes away from that intensity, and sometimes the anime's humour was just straight up goofy (and while I personally enjoyed the goofiness, I can see why Inoue may not have liked it and decided to pull the plug on the anime).

Also, Miyagi's backstory doesn't just "come out of nowhere". A couple of years after finishing the Slam Dunk manga, Inoue wrote a short story called 'Piercing' where the main characters are two young kids named Ryota and Ayako who happen to look like Miyagi and Ayako. When asked whether Piercing was canon to Slam Dunk or had nothing to do with it apart from the resemblance of the characters, Inoue just said "I don't know, I'll let the fans decide if it's canon or not". Well, the movie went and officially made Piercing canon through Miyagi's backstory.

Overall, even though the movie's very different from the original anime that I know and love, I still thought it was a masterpiece. It adapted the match very well from the manga, it adapted the Piercing story beautifully, and to me it also even matched the theme of the series illustration books. I personally see melancholy as the top theme that Inoue likes to work with as an author (you can see that even more well with REAL) and I just think he's masterful with it.

It's not like melancholy is absent from the anime either, I encourage you to go back and watch the old Slam Dunk movies from the 90s - the plot of one of them is literally that Rukawa's successor as the ace of his middle school team (and soon-to-be Shohoku student who dreams of playing in the nationals with Rukawa) ends up getting diagnosed with tuberculosis and begs Rukawa to play one last game with him before he retires from basketball.