Other pertinent data point, how many epidemics have been positively traced to a lab leak since virology has been widely studied? The Wuhan lab was founded in the 1950s. You can say the likelihood that a virus would one day escape from one of these labs is pretty high. The likelihood that a given virus would be from a lab is very low. All of which brings us back to where we were at the start. It's plausible and possible but not really likely.
A 2007 outbreak of foot-and-mouth disease (an important virus affecting cattle) was traced to effluent released from a laboratory in the UK [1].
A small number of SARS infections in 2003-2004 are also believed to have been due to laboratory accidents [2].
This article [3] gives an introduction to the subject from the perspective of a journalist who has reported on laboratory safety in the US.
This article [4] published in Nature in January 2012 by members of the US National Science Advisory Board for Biosecurity reviews the risk of a release of an engineered form of H5N1 influenza. It includes some alarming remarks such as:
'We found the potential risk of public harm to be of unusually high magnitude' and;
'A pandemic, or the deliberate release of a transmissible highly pathogenic influenza A/H5N1 virus, would be an unimaginable catastrophe for which the world is currently inadequately prepared'
The authors take the possibility of release of a dangerous pathogen from a laboratory seriously, though the article is prospective rather than retrospective.
For an epidemic to occur, you need not just a lab leak, but a population sufficiently naive to the pathogen. H1N1 was displaced by H2N2 in the late 1950's pandemic, which in turn was displaced by H3N2 in the late 1960s pandemic. Thus it hit the cohort of people aged 25-6 or less who'd never been exposed to H1N1.
That article doesn't support your argument. It just says it was suspected.
I found an NIH article that says the likelier origin is that the 1950 virus was used to produce a weakened live virus vaccine candidate that lead to the reemergence and not an accidental leak. It also concludes by saying there has never been a likely lab leak epidemic ever observed.
That article's definition of "lab accident" seems narrow and legalistic to me. In either case, the virus spent 1950-1977 in a lab freezer. It ended up in the wild, with ~700k people dead. The only question is whether it escaped in an infected researcher (or in infectious lab waste, or in whatever else you'd consider a proper lab accident) vs. in that failed vaccine candidate.
Those details do inform some details of the correct policy response. For example, they determine the relative importance of better PPE at the bench vs. better QA before allowing the vaccine to leave the lab. They don't change the overall question of whether scientific research has ever caused a pandemic, though. That causality is what matters, not whether the sign on the door said "lab" vs. "experimental vaccine nurse".
For example, if the pandemic originated from a WIV researcher who became infected in the field (during their many expeditions to remote bat caves that no other humans would routinely enter), was that a "lab leak"? Literally no, since they weren't in the lab. The causality would still be the same, though--if not for that scientific research, that virus would likely have never left the cave.
To avoid such confusion, it's probably better to say something like "unnatural origin", or "origin arising from scientific research". A much bigger mouthful than "lab leak", though.
I'd add that the article does not state that there have never been cases of accidental releases of pathogens from laboratories, only that such accidents had likely not led to a 'global epidemic' as of the date the article was written (2015).
The article's abstract opens with the statement 'The 1977-1978 influenza epidemic was probably not a natural event'.
