A recent situation that I found myself involved in has motivated me to write a post about a topic that I've been meaning to write about for quite some time. This is the assumption that high-end components, or even frame sets for that matter, are supposed to last forever (or at least a very long time). For the most part, this couldn't be further from the truth. Components such as Shimano Dura Ace and Campagnolo Record do command a price that says that those said parts should +10K miles, and for the most part, they do. But a couple of points that I want to make clear is that that Dura Ace or Record Carbon rear derailleur that has +5k miles on it will not work like it did when it was new. Normal wear on a component will cause a downward sloping curve in regards to performance. More use causes more wear which will negatively effect the performance. This degradation of performance is usually the result of wear of the pivots and other parts make contact with each other. Besides normal wear, contamination in the form of abrasives such as road grit and salt, also have a major influence on how the components function. I won't even factor in damage from crashes and abuse.
At this point I'm sure that you're wondering if getting higher-end components are worth it. This can only be answered on a case-by-case basis. If you want the best performance that the industry has to offer, then, yes it's worth it. Again, this is dependent on your specific needs or wants. Is Dura Ace is better that Ultegra? The quick and dirty answer is yes. Will a XTR front derailleur better than a XT? Again, yes. With all of this being said, the point that I want to make is that when you buy a Dura Ace or XTR rear derailleur, you are getting a professional level rear derailleur that are used by the sponsored pros but are replaced at LEAST once a season which is usually much less than a year. Pro-level bikes are sent out to pasture after the racing season ends and for good reasons. The bottom line is that Dura Ace will last longer than Ultegra and Ultegra will outlast 105, but they will not last "forever" and function like new after thousands on miles of use. And let's not forget that these incredible components still need to be maintained. Thanks for reading.