To make the argument that, because in the past the transmission from animal to human happened in nature, that in the present it could only happen there, implies

To make the argument that, because in the past the transmission from animal to human happened in nature, that in the present it could only happen there, implies that your assumptions stay constant. Things have changed since Ebola 50 years ago and Sars1 20 years ago. For one thing, millions of dollars have been invested in gain-of-function research. There are many more labs, many times more investigators and projects running. These scientists are working to create the characteristics in viruses that make them more deadly and more infectious. Also, today powerful new bio-engineering tools are available, in the past 10 years especially. Today the situation is different, since there are now new pathways for a deadly virus to come into being. I don't think anyone can argue that the number of dangerous viruses being studied in laboratories around the world is the same now as it was 20 years ago. If you want to argue against a lab-leak today, you really need to argue that the BSL-rated laboratories are perfectly leak-proof.