Cars wear down and get worse over time. A computer does not.
Not disagreeing with the general concept (I intend to keep my 2009 for a long time), but let's talk about...
-electromigration
If a certain processor isn't designed correctly, the conductive surfaces will deform. AFAIK, it only takes one transistor failure to kill a processor, but I could be wrong. In addition, a nasty voltage spike could (in addition to damaging other components) also lead a poorly-produced processor (one in every batch!) to slowly decrease its clockspeed.
-capacitors
The companies that produce parts for Apple know that they need to use components of a particular caliber (otherwise Apple loses money with RMAs), but every now and then a capacitor will fail and/or stop working - aka "popped caps". You can replace these yourself, although there's a chemical hazard involved (plus you can get better caps and replace them yourself)
-heat dissipation
Don't live in a dusty environment. If you do, tear apart your machine on a regular basis to clean out the heatsinks. Not to mention if a coolant is used, failure at least results in a mess depending on the material or at most can kill a machine. Not to mention thermal paste can dry out, which needs replacing.
-hard drives
They last a long time. But there are bad batches that eventually lead to bad blocks (almost every producer has had a massively bad batch in the past). Consistent failures mean that your computer needs a new hard drive.
-warping
A motherboard can warp over time, sometimes making RAM banks useless. etc.
I'm not disagreeing that your computer is going to go any slower (unless you get a lot of dust in your heatsinks). Its just that a computer does need maintenance from time to time, much like a car. I -hate- car analogies, but cleaning is like changing the oil filter, yada yada. Its why computer janitors exist.
tl;dr: Spergin' about computer maintenance.