Arguably not, though he forever remains in the top three after Druitt and Tumblety.
From one angle, "Kosminski" remains a strong, contemporaneous suspect because he was the choice--in fact the solution--of Sir Robert Anderson and Donald Swanson.
Too much of the little they wrote either does not match Aaron Kosminski, or makes no sense in terms of other policemen being ignorant of this suspect's extraordinary bona fides (or even of his existence).
Furthermore, Macnaghten, arguably, can be shown to know more accurate data about this suspect and he absolutely rejected him as 'Jack'. I further argue that this suspect's elevation, his being sexed-up, originated with Macnaghten and was passed on to Anderson.
Because of RipperLand's suffocatingly narrow focus on the Mac Report (official version) it is not well known or understood here that Macnaghten in 1913 and 1914--just like Anderson in 1910--claimed in public that the case was solved, but went with the man who who had taken his own life.
To make a convincing case for Anderson and/or Swanson as the most reliable source(s) of the time, and thus the Polish madman as the likeliest suspect, you have to explain away the above.
Yes he is, and the DNA stuff, right or wrong, gives everyone another chance to look at him and to see that any of the mistakes made by Swanson and Anderson are minimal and easily dismissed as coming from a time when there was no Google that helped with coroborration. I still like Kelly better, but I have nothing but me 'spicions.
Don't tell Fish or Lech, but I'd put Crossmere higher than Kosi. The whole Anderson thing seems a little too anti-Semitic for me. They are all "persons of interest" but all lack actual evidence so far.
"Whenever an expert says something that bolsters the Lechmere theory, it is not my task to disprove him ..." Fisherman
For once I am hardly alone in writing that the broad argument for Kosminski, which, fair enough, becomes an argument for the veracity of Anderson and Swanson, is terminally weak.
The errors that Anderson and Swanson made were, we are always unconvincingly assured, 'minimal'.
As in nothing to see here, keep moving folks.
Minimal ... Really?
1. They have 'Kosminski', no first name, deceased soon after being incarcerated which they imply--Anderson several times-- happened in early 1889. In fact, Anderson predeceased Kosminski by a few months. This is why Martin Fido was not looking for the incarceration of a Kosminski as late as 1891, and partly why the writer/academic rejected this Pole as Anderson's suspect once having stumbled upon him.
2. Swanson writes and Anderson implies that no murders after Kelly were thought by the police to be by the same killer. A quick look at the sources from 1888 through to 1891--and arguably even as late as 1895-- disposes of that one. Especially the agitation over Tom Sadler and Frances Coles.
3. There was no witness identification by a Jew of a Jew, at the police seaside hospital or anywhere else, because Smith and Macnaghten would have to have known about it. In different styles the latter both firmly reject the existence of such an encounter. Buttressing that rejection is the 1908 interview which shows Anderson's alarming capacity to make a dog's breakfast of the past, and therefore the Evans-Rumbelow 2006 theory of Kosminski [self-servingly] substituted for Sadler makes sense of the little we have.
4. Even more devastating for this line of argument--so it is never dealt with--is that Sir Melville Macnaghten knew more accurate data about 'Kosminski' than Anderson and Swanson put together. He knew the Polish suspect was still alive and he knew that he was out and about for a considerable time after the Kelly murder. In Guy Logan's 1905 semi-fictional account of the Druitt solution, Kosminski is eliminated altogether, as he was to be in Sir Melville's memoir nine years later, e.g. he was judged to be nothing. 'Kosminski', with the same first name lacking, begins with Macnaghten in the extant record when he backdates the former's incarceration to March 1889 (matching Anderson's memoir and the implied dating of the Swanson jottings) but with Sims in 1907 this becomes a lengthier interregnum--plus Kosminski is further fictionalized (he lives alone, worked in a Polish hospital, was sectioned by the state).
4. In the primary sources of the Late Victorian Era and Edwardian Era the Polish madman was strictly a sideshow to the main game--the Ripper as a tormented, English gent. Therefore some modern writers are overturning the received wisdom of the past. Nothing wrong with that. It is done all the time. My problem is that it is not acknowledged it is being done, and therefore it is not understood how much has to be upended and recalibrated to make this revisionist interpretation work. Instead we have bland reassurances by some that it is all quite straight-forward re: Anderson-Swanson to Kosminski.
It is anything but straight-forward, and the perplexing features of Anderson's claim was perceived and understood at the time, e.g. as perplexing and even implausible, but far less so now.
This is how theory becomes doctrinal before calcifying into dogma.
Could Aaron Kosminski have been 'Jack the Ripper' after all?
It's possible. Anything is possible, but it is just not very probable.
Why not? Because a competent, hands-on police chief of the day rejected this suspect. Why choose his opinion over the others? Because he can be shown to know more that is accurate about the same suspect, and why shouldn't he as he was there.
On the other hand, Martin Fido and Scott Nelson arguably make a better case for David Cohen as 'Jack the Ripper'--even if we leave out the competing police chiefs.
Since no one is known to have witnessed the Ripper in the act of actually killing someone, Kosminski can only be suspected by implication. And if the witness who supposedly identified Kosminski was Israel Schwartz, as I believe it was, he could only tie Kosminski into the assault on Stride in the street outside Dutfield's yard 15 minutes before her body was found - not to Stride's murder. Swanson, in discussing the identification of Kosminski, simply made a logical assumption that if Kosminski wrestled with Stride in the street, he must also have been the one who killed her and, by implication, all the other Ripper victims.
Thus, in truth, there is no tangible evidence connecting Kosminski with any of the Ripper murders.
"We reach. We grasp. And what is left at the end? A shadow."
Sherlock Holmes, The Retired Colourman