I wouldn't say abruptly, everything was solved in Slam Dunk, there was not plot holes IMO and I prefer the ending of a series before it gets repetitive. Haikyuu has a clear objective and by the recent development, it is not far from the solution, 2~3 years is more than enough to end the series.
Not everything was solved. Inoue had already created a ton of new teams and new players and didn't used them. Also the fact that Sakuragi the main character growth in basketball stopped in the middle isn't something that can be called "solved".
Something not being solved by the end of the series isn't synonym of plot hole! So yes, Slam Dunk doesn't have any plot holes, but it doesn't solved everything.
What clear objective are you talking?
- Kageyama objective is being the best in the world
- Hinata objective is surpassing him in the world level.
- Sawamura and Karasuno objective was until 2 chapters ago, just going to the Spring Nationals. Now Sawamura set them for "winning the nationals", but as was said is obvious that if you go there you must go to win. So is not like their major objective is winning the nationals.
Then you have
- Yachi as a the main team manager.
- Ennoshita as the next year captain, and his improvement in defense.
- Datekou battle against Karasuno and with it the newbie setter(that if he manga ends as you say he just appeared to disappear), next year.
- Seijou new team, with 2nd year setter and Mad Dog, next year.
- The Shiratorizawa 1st year that wants to be ace, next year!
- 2 meter guy evolution, from a total amateur to ???
- Oikawa after Highschool. So College or Professional. And there he wants to fight against Ushijima Wakatoshi and Kageyama.
- Ushijima Wakatoshi already went to Japan sub-18. So increases the possibility of a world competition existence.
- Takeru and Hinata fixation to the Small Giant, that is right now in College or Professional.
There is still a lot of content that the series has to show. The author can just go and end the manga in the 1st year tournament and leave a lot of stuff behind, or he can just continue after that year and show more stuff, stuff that was already referenced and developed.