If the testimony of the dying man is sufficient for probable cause for an arrest (and it is) then the same testimony should be admissible in court. As to the Confrontation Clause there is no real issue. Though the witness is not available for cross-examination his testimony can still be challenged by the defense. Evidence could be produced (if it exists) that impugns the witness’ integrity, eyesight, disinterestedness.
I don’t think people are arguing that such testimony would be sufficient by itself to convict but the jury should be able to assign the proper weight to it.
And what if such dispositive evidence doesn't exist. What if the credentials of your dying person are unimpeachable, yet, for whatever reason, he was mistaken in what he saw or heard. Such an error can not be extracted from third party testimony. Only through direct examination of the witness.