Kise didn't say that Kagami and Kuroko will for sure have to part way. He said that if the only thing Kuroko appreciate about Kagami is his attitude toward basketball then as soon as his attitude change, Kuroko will no longer appreciate him and they'll have to part way.
And he's right, because Kuroko and Aomine's bond was based solely on basketball and when they lost it, they stopped seeing each other as well. They said it themselves, outside of basketball they had nothing in common or great chemistry, they would not have been friends if they did not have the same passion. Sure, the care they have for each other and their bond is still important and real, but without basketball between them, Aomine and Kuroko had nothing to bond them together and they drifted apart.
When Aomine's attitude towards basketball changed, Kuroko no longer enjoyed spending time with him. Yes, Aomine abandoned Kuroko on the court when he grew strong enough, but we often forget that Kuroko abandoned Aomine off the court as well.
Kuroko is a victim like all miracles, but like them, he is not without faults either. And he knows that, he'll be the first to try and correct his mistakes and regrets.
When Kise sees for himself that the bond between Kagami and Kuroko already goes beyond basketball, he is reassured. He understands that the same thing won't happen, that even if Kagami changes, Kuroko will stays by his side and stop him.
Also Kuroko's 'look at these biceps' and 'yeah i forgot to think ahead', he's not even trying to be funny but his blunt honesty is hilarious.