"Most of mine have been FrankenPCs, put together from parts."
Same here - at the last count about 50 "new" PCs since 1993. Recycling PC bits usually hits a brick wall when there is a significant change in architecture eg AT, ATX, ISA, VESA, PCI, PCI-e, RLL, IDE, SATA, serial ports, parallel ports, USB1/2/3, SIMMs, DIMMs, DDR, DDR2, DDR3, AT psu, ATX 20pin + 4 psu, ATX 24pin +4 +8 psu.
Re-using hardware peripherals can also be inhibited by the lack of drivers for a new OS version.
My garage is full of obsolescent bits. There are ten ivory PC ATX cases with "small" case fans. Last time round everyone wanted black and water cooling with big case fans
The major driver for my upgrade cycles was usually when the "godchildren" wanted faster graphics for a new game. They seem to have grown out of that "must have the latest" phase.
For other uses it is the licensing of software that causes the headaches. The thorny question is always whether a test upgrade onto an experimental tech refresh PC will lose the legal licence of some essential, expensive package?
The last crop of tech refreshed PCs was about three/four years ago. Asus P7P55D variants plus Nvidia 260, 275, and 280 graphics with 500gb drives and dual boot XP + Vista/W7 64. One godson skipped on that round - so his XP P4-D now needs an upgrade - probably W7 only on a P7P55D-E (second-hand) with Nvidia 570 or 660. It is anticipated that the latter won't need the motherboard to support PCI-e 3.0.
The laptops are XP Dell 5150s - mostly bought second hand. They will be replaced by second hand W7 Dell E6410 and probably Libre Office.
Most of the godchildren are now earning enough to have bought themselves Apple laptops for phone app development or music composing.