That first part isn't right. Doom was entirely CPU-bound software rendering, and 3d rendering cards didn't exist yet. You couldn't upgrade a 386 or 486 PC to play Doom better, short of replacing the entire motherboard with a new CPU platform.
What you're saying was fairly true by the time of QuakeGL and Quake 2 around 1997, but not Doom in 1993.
> You couldn't upgrade a 386 or 486 PC to play Doom better, short of replacing the entire motherboard with a new CPU platform.
Yes, you could; CPU+chipset (and, in some cases, + RAM, IIRC) processor upgrades which plugged into an expansion card (to power some of the support hardware) and the processor socket existed.
No amount of VGA, ram or HD upgrade would help with Doom. The difference between absolutely the worst 8bit ISA VGA cards (Realtek, RTG) and one of the fastest 16bit ones (ET4000) was 1-2fps on 386DX40, a whooping 10-30% of overall unplayable unless you are really stubborn <10fps performance.
The way you made Doom playable was swapping motherboard/cpu/ram/vga for a 40-66MHz 486/localbus ones at a cost of multiple A1200s, not exactly what one would call 'extended pretty cheaply'.
What you're saying was fairly true by the time of QuakeGL and Quake 2 around 1997, but not Doom in 1993.