Changing agencies every two years

Via Strategic Public Relations, I read an article stating that most clients change their marketing/ad/PR agencies every two years.

I have worked in PR for many years, so this was relevant to me. I guess in a way it was reassuring to note that even agencies, not just one-woman-shops like myself, deal with client churn. It's the part I hate most about my current work situation -- the constant trolling for work. It's the part of my job I can't imagine doing into my 50s and 60s. If someone else just handed me work all the time, but I still got to keep my freelancer status, earn the same rate, work my own hours etc., that would be my ideal. THAT I could do for another 25 years.

Unfortunately, life doesn't work like that. And this article bears out that it is pretty impossible to do agency or freelance work without the continual search for new work.