My experience is different. I too have been mining on dozens of 5970s, continuously, with the fan speed manually forced at 100%, with my oldest cards mining for 18+ months. I would estimate my fan failure rate is at least 25%. I don't know the reason for the difference, but if I were starting over now (say with 7990s) I would definitely try to take it easier on the fans.
Dust is probably the root cause of your fan failures (are you mining at home?). My miners run in a datacenter with filtered air. When I have compared my low fan failure rate to other 5970 miners, dust has always turned out to be the main difference between my environment and theirs. Your fans would have likely failed just as spectacularly even when running at lower speed...
Also, most of my fan failures affect disproportionately 5970s, not 6990s, even when taking into account the fact that my 5970s are generally older than 6990s (I have 5970s whose fan failed despite having been in operation for less time than most of my 6990s). I would venture that the fan on 5970s is of lower quality than 6990s.
My cards are in a data center. A few of the earliest ones were run at home, and a few were purchased used with who knows what history, but many of the failures were new cards run exclusively in the data center.
My cards are in cases and densely packed with something jammed between them to create small gaps (similar to your blog post but probably even smaller gaps due to the case). Perhaps heat is the issue, or perhaps the restricted air flow makes the fans work harder. They are also 5970s which may be worse than 6990s.