Yeah, I have had the same experience (although I have not been using them as long as you have). I have also read the same thing on other forums.
My alternative is to use a good quality synthetic for most painting, such as base coating, blocking, etc. My synthetic of choice are Raphael Kaerells. They are half the price of Series 7s. My Series 7s are reserved for tiniest detail.
You could also try Isabey sable brushes. They seem to hold together pretty well.
By the way, I don't buy brushes online.