If there were an eyewitness in the O.J. Simpson case there might have been a conviction. All the scientific evidence simply gives probabability that a person might have been at the location at some time or another.
Currently, DNA evidence can prove what is not possible in the area of paternity and with very close matches can prove paternity. Hair, clothing, footprints, shoe prints and wear can be matched precisely using topographical mapping techniques with the aid of computers.
But, in our court system, all scientific evidence can be skewed and interpreted in a variety of ways by "expert witnesses". Evidence can be contaminated at the crime scene or in the lab by human error as well. If there is an actual eyewitness who can accurately identify the perpetrator of a crime who is willing to testify; that is hard evidence to refute. At some point in the future, science may advance to the point that it provides sufficient evidence to prove guilt or innocence, but that day is not here yet.