At no point will any fewer cattle be 'processed' for meat because of this decision.
What does change is the utility ratio of the beasts. Well use less of each beast making the tragedy of their death more meaningless.
Leather is far more environmentally friendly than plastics, with a small caveat for the tanning process' chemicals, and emissions from the beasts themselves (though that's attributable to beef production.)
Leather doesn't degrade into micro plastics.
So unless Apple is also reducing its beef consumption* by the equivalent amount it's pointless.
Making cows(not beasts) less viable to grow because they are more expensive because the farmers aren't getting money for their other body parts is a win!
Less leather bought = more expensive cows = less people able to afford cows = less cows murdered.
Yes, the person did take that into account. That was the whole point. If leather can’t be sold, meat prices will have to become higher (if farmers are not to get less money per cow). This will lower demand for meat, and less cattle will be raised.