If I remember correctly I believe he has also always been, even after all these years, strictly a hired musician. This was brought out as part of one of the Sammy articles I read last year. Michael Anthony doesn't necessarily participate in the songwriting/publishing/production etc. parts of the business and needs to kind of go with the flow.
On a personal opinion basis, I also consider him one of the luckiest guys in Rock n' Roll to have hooked up with who he hooked up with. Being in the right place at the right time can work wonders.