Why does Paul say that David didn't go to Heaven in Acts 2?
To your question ...
This has nothing to do with whether or not David would be with God forever, but was instead a realistic observation that David had written about someone other than himself. You see, David's words seemed to indicate that he was referring to himself.
For David says of Him, I SAW THE LORD ALWAYS IN MY PRESENCE; FOR HE IS AT MY RIGHT HAND, SO THAT I WILL NOT BE SHAKEN. THEREFORE MY HEART WAS GLAD AND MY TONGUE EXULTED; MOREOVER MY FLESH ALSO WILL LIVE IN HOPE; BECAUSE YOU WILL NOT ABANDON MY SOUL TO HADES,NOR ALLOW YOUR HOLY ONE TO UNDERGO DECAY. YOU HAVE MADE KNOWN TO ME THE WAYS OF LIFE; YOU WILL MAKE ME FULL OF GLADNESS WITH YOUR PRESENCE.'Acts 2:25-28
But the truth is that what he wrote referred to someone far beyond himself. The people could visit David's tomb and figure out for themselves that his body hadn't gone anywhere, but had remained to rot like everybody else. But Jesus HAD risen from the dead so that HIS body would never rot.
Once again, this is not saying that David's real life hasn't continued on with God, but is instead giving the basis upon which he, and the rest of us, can be one with God!!