Wrong.
Those PC's are useless, and nothing you say is true. The only advantage of PC "modularity" is for manufacturers to ship more models, which is not a problem for Apple, specially in this day and age.
"Upgrading" a PC is one of those things that got a taboo with enough push from PC "enthusiasts", salesmen, hardware manufacturers, etc...
In real therms, upgrading a single part on a system, is just stupid and economically unworthy.
If you want more performance, you can't get it just by upgrading a CPU, or RAM, or whatever. You can only get it by upgrade the WHOLE machine.
There's no performance gain in a system that runs ok with 8GB to move to 16GB or 32GB, if that memory is STILL slower, if the CPU can't access the RAM as fast.
When CPU's change appreciably, the chipsets and sockets also change, so, you won't get a radically better CPU, you get another range from the same CPU, you need to buy a radically better CPU to appreciate the difference, there's no way you'll tell the difference from a couple of MHz, yes, there are dual core, quad-core, different clock's, but that's just a small difference from generation to generation, and the best thing to do is to buy the correct CPU for your needs.
Seymour Cray