The other thing about the DNA testing of the letters is that they're bound to be contaminated by others anyway, including (but not limited to) the lead detective on the case. And (iirc) the handwriting on some of the letters is different (and one of them is thought, but not proven, to be the work of a hoaxer).
All in all, it's unlikely that anyone will ever come up with
the definitive answer, not when serving and former Scotland Yard detectives - all of whom have cracked tough cases in their careers - as well as forensic scientists with a specific interest in the case have got nowhere near to solving it.
(As for the amateur 'Ripperologists', for every one that's worth listening to there are at least a dozen who aren't, and who have come up with the most far-fetched ideas and 'proofs' to back up their - usually heavily agenda-based - argument. For example, one 'theory' still has it that the Ripper was the-then Crown Prince, who was diseased and blamed prostitutes particularly - and women generally - for his condition; the Yard cleared it up - the case, not his dose of whatever
- and identified the killer, but it was hushed up and the detectives on the case, silenced. That one's really popular among Republicans (UK and Commonwealth, not US
)).