Solid state technology is much more reliable than the mechanical components of a car; especially over it's lifetime (cars are expected to last > 10 years, computers < 5). Those constraints lead to vastly different optimizations made in the design.
When I was a teenager I made reasonable pocket money upgrading peoples computers from 16 to 32/64MB of RAM because it saved quite a bit vs the cost of buying a whole new computer and made a significant difference when running Windows 98.
Your perspective on auto-mobiles is bizarrely limited. Of course people upgrade their cars... There is an entire industry built around doing just that.
The performance part industry caters to very specific niche of users analogously to gamers in the PC industry. This niche should avoid iMacs.
That new stereo the neighbour kid has? Upgrade. Those new Xeon headlamps that jackass coming the other way on the country road has? Upgrade. Those new windshield wipers you got last year that don't totally suck? Upgrade. Those big rims you see on cars at the local 7-11 once in a while? Upgrade. Just about every single jeep wrangler you have ever seen? Five thousand upgrades. There is a whole class of car for which it is more rare to see a stock specimen than modified. Check out a used car magazine sometime. The number of companies just making aftermarket suspension kits alone for specific models of Jeeps is absurd.
The aftermarket industry for automobiles is massive, and the automotive industry for the most part accommodates it very nicely. This is because, presumably, they are not comfortable with marginalizing what you seem to describe as an insignificant minority. Car enthusiasts.
I can see why the automotive industry may be unfamiliar to a lot of HNers (Lots of us live in cities (hell, these days my car is lucky if I use it once a month), spend our time avoiding grease, and have large salaries so our cars represent a relatively small or otherwise inconsequential investment). But just because it is fairly invisible to you does not mean it does not exist.
A RAM upgrade isn't about a RAM replacement because of failure. It's because as computing progresses, you need more RAM to remain performant. A RAM upgrade is the single most effective way to revitalize older hardware - that 8GB that is more than enough today may be not nearly enough tomorrow.