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.