I have two Logitech MouseMan Dual mice which I bought almost 10 years ago. This model was far more expensive and provided a vastly superior usability than its competitors and I still love it, but time went by and its mechanics failed.
What do I mean by "mechanics"? Well, considering that every recent mice use purely light (either optical or laser) for motion tracking, the only mechanics left in mice are microswitches. These switches are usually provided by OMRON and most of their switches can endure about 10 million operations which may sound much but I guarantee you that it won't last more than several years of intensive usage. What I want to conclude is that the current design of mice is potentially incredibly durable, except the microswitches.
Given the kind of green person I am and considering how much I hate planned obsolescence, I was thinking on how is it possible to maximize the lifetime of a mouse with minimal resources. The operations below require a soldering station and a modest amount of soldering skills (or a friend who has the equipment and is willing to do the work for a beer).
1) Switch swap
Switches do not completely die from one moment to another in my experience. Usually you notice that drag and drop operations are hard to make because the switch cannot operate correctly during sustained action. Once you think about drag and drop and you realize that only the left mouse button is used for it and others are not used for sustained operations you immediately ask the question: why not swap them?
The middle and right buttons are very rarely used compared to the left button so they can last about 10x (right button) to 100x (middle button) longer. First, you can swap the left and middle buttons and several years later you can swap the (then) left and right buttons. This should give you alone a 3x durability for no money!
2) Buying a new switch
Once it gets inevitable you have to buy a new microswitch eventually. The challenge is that the exact switch model you need is probably not in the market by that time because the manufacturer have obsoleted it. However, it's probably possible to find another switch with almost identical dimensional and mechanical characteristics, so keep looking! I'd really love to see a site listing all the microswitches that are on the market for various mouse models.
3) Designing a mice for durability
Even though mouse manufacturers have no financial incentive to design a mouse for a lifetime, it's worthwile to think about the issue. Putting the microswitches into sockets for easy replaceability and providing replacement switches for sale and/or bundling a dozen of them with the mouse itself would solve the issue.
Enjoy and make your mouse last forever!