> marketing overtook technological improvement in product development

I would say another key change is things just becoming less modular over time.

Like, the chipset used to be a major factor in choosing your motherboard, but it just doesn't matter anymore. Third-party chipsets are no longer a thing, and there's little difference between first-party chipsets anymore because every CPU has a full integrated northbridge now.

And honestly, today's PCs are powerful enough that there's no point in even bothering to make optimal choices. You could pick mediocre parts for all your stuff and still end up with a beast. It's not like the P4 or Athlon XP days where you'd feel it if you picked a bleh motherboard or something.