But if all the members leave then there will be no one to read the ads.

This argument can go on ad infinitum and did so a couple of months back but I can't be bothered linking to the numerous threads about it.

There were many threads about the dropping membership on this site and then the upgrade to vBulletin 3 took over and there were many issues, of which one was the advertising width etc.

It was fixed for a while, but now it's back but in the interim more people have moved on or visit less.