The minute someone falters and they can't do the job like before anymore, it doesn't matter how reliable you were in the past, how much you did for the band, you're not cutting it lately so out you go.
One one hand, as a business, I can understand that. On the other hand it doesn't seem like his mates want to help him, they seem like they are leaving it all up to Phil. I sort of understand, no one is their brothers keeper, but still it seems something is missing. Loyalty maybe. When you laugh the world laughs with you but when you have problems and aren't performing up to par, tough crap. NEXT!
AC/DC are as much of a business brand as they are a band now. If one of the members isn't able to fulfil the most lucrative part of that brand, ie touring, then time to get a new person to do it.
If Phil gets convicted of possessing drugs he won't be able to enter the US and his time in the band would be over anyway.
As someone who has spent time detoxing drug addicts (on the assumption that Phil has a meth problem, it was certainly found in his house)there's just about nothing you can do to help until the person accepts that they want to change their life. Sometimes the best and kindest thing to do is to cut someone loose. Used to have to explain this to families on a regular basis.