That is simply BS. There have been many cases. But if you chose to ignore the facts then more power to you.You can take DNA from the dead. We do it all the time. If there was any chance someone was executed by mistake, and DNA would have proven that, it is unacceptable that we would not try to exonerate that person. No. There is no indication that anyone was ever executed by mistake.
2 people in Texas last year were released from death row due to DNA evidence. It might pay to know the argument before you open your mouth.
And in case you need some help with simple logic, having the DNA of a dead man does not prove anyhitng unless the DNA of teh criminal was also collected to compare it to. So your whole rationale is flawed.