Science will never eliminate the need for eyewitnesses. The trouble with science is that, as much as it can provide "proof", it can be interpreted in many ways. Currently, in the case of toddler Caylee Anthony, forensic scientists are disagreeing about the duct tape found on the skull. Some are saying that the presence of certain things prove that the duct tape - i.e. suffocation - is the cause of death. Others are saying that there isn't enough evidence on the tape to prove any such thing.
Even DNA evidence has a element of doubt - it is just that, in most cases, the doubt is so small that scientists accept the findings, and then verify such findings to the jury. Most cases involving scientific evidence have different "experts" on that evidence arguing different things. Therefore, eyewitnesses - although themselves open to doubt - are definitely needed to help build a case of guilt or innocence.
One of the most corrupted legal systems that ever existed was in the mid 1850's London courts. It was there where some of the most absurd eyewitness accounts occurred and where most innocent people went to jail for crimes against morality, because someone "had seen them", even in an era where modern optometry was not available, half the town was possibly near or far sighted, and the effects of syphillis on the eyesight of many were rife.
The problem I have with eyewitness accounts is that our brains have become less able to focus on detail and, being that our society has granted us so many instances of instant gratification and quicker, faster, and more available information, I fear that we do not take the time anymore to really look at individual characteristics of people. Also, we live in a very hyped society where everyone has to be extra sensitive to not be a victim of crime. Could that hypersensitivity, that lack of focus, and the overload of TV episodes that deal with crime be affecting our natural ability to discern what we see from what we think we see?
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.