The cause of many of these bugs is an aggressive annual release cycle and fixed release deadlines, not necessarily insufficient QA. Making a significant part of the QA function voluntary wouldn't help with meeting tight deadlines or dependability of the QA process.

The general point of expanding QA resources might help, but it's certainly not a given that making that on a voluntary, best effort, when you get round to it basis would help with the timeliness and reliability of releases without sacrificing some of the other goals Apple and their customers holds dear.