DeNiro's screen test as Sonny was electrifying--I never tire of watching it. But DeNiro played Sonny in much the same way he played Johnny Boy in "Mean Streets" (which he'd recently finished for Scorsese). His Sonny would have been a very different one than Caan played. And then we'd be left with the problem of who would play young Vito in II.
As you probably know, DeNiro was considered for Paulie in GF. Imagine if he'd gotten that tiny part--and not young Vito.