I joined Audible in 2006 when it was 5 books a month for $20. I also got a free Rio 500 for joining, which cost $350 in the stores at that time. I quickly got a second account so I could get another free Rio 500. I did tell them what I was doing and they were okay with it.
I kept 2 accounts going for about 10 years, getting 10 books a month. I never had any problems finding enough books. I never listened to that many a month but I was retired and I wasn't sure if I might end up in a nursing home with the health issues I was having then and I was buying for the future.
They have such a huge selection that I never had enough credits to get all the books I would like to have bought.
I did cancel a few years ago but all of my books are still available. I've never had anything but good feelings about Audible. They make their options plain. I always knew what I could and couldn't do. If I screwed up they always found a way to make it okay.
My guess is that this thing about losing credits is one of the ways they keep the cost of the plan down.
Barry