People who don't believe in a Ripper generally try to pin the murders they dispute as one-offs by people who knew the victim - as is on display in this very thread. So I think even they tend to associate serial killers with stranger-killing.
"Serial" killings consist of 2 or more murders, so in that context is it highly unlikely that within some 13 unsolved cases there might be multiple, multiple murderers, as well as individual murders?
Personally I believe that there was someone who fits this Jack moniker, in that he was a monster who killed and mutilated simply because he desired to. I think he killed at the very least 2 women within the Canonical Group. The circumstances were the same, as was the Victim profile, the method used to subdue then incapacitate seem virtually identical, the wound patterns show the same unusual focus and severity, and both were left to be discovered shortly after the crime was committed. Which adds the ingredient of Terrorist and perhaps voyeur, if he chose to return to the crime scenes shortly thereafter, within the crowds.
Its not an objection to the premise that there were serial murders during that period, I accept that there was one... and likely 2, if the Torsos were executed by someone else.
Its an objection to the premise that the very different circumstances in each case and the very different types of attacks can be explained by a singular serial monster who apparently owned the murder franchise rights within greater London for a number of months, or perhaps years.