Vegetarianism today

In honor of the Rabbi:
The rabbi wrote (https://mikyab.net/posts/2357#_ftn1; https://mikyab.net/%D7%A9%D7%95%D7%AA/%D7%A2%D7%9C-%D7%98%D7%91%D7%A2%D7%95%D7%A0%D7%95%D7%AA) that nowadays there is a complete prohibition on assisting the animal food industry.
Why is the IZ included in the definition of the Poskim (1744, 315; Ramat Abe’a 315) that tseb’ah for a need is permitted, and since today’s need is greater (quantitatively), the industrial route is defined as tseb’ah for a need?
Furthermore, according to the Rabbi, why is there no prohibition on buying in places that sell meat, when in fact meat is indirectly sold to people (despite the Poskim’s view that the prohibition of tseb’ah includes an active command to prevent harm (the Mitzvah (Mitzvah P’) linked this to the dispute over whether tseb’ah is from the Torah or from the Rabbis)).
Thank you very much!