I am a bit puzzled by this:
"Before about the early 90s, we didn’t trust C compilers, so we wrote in assembly."
There a lot of games released during the 80's, were they really all written in assembly ?
https://www.myabandonware.com/browse/year/
I don't have experience in the game industry at all, I must add.
Heavier things, like graphics processing were typically written in assembler. Or sound mixing.
Some things like texture mapping you could only write in assembler, because you'd need to use x86 lower/higher half of word (like AL and AH registers) due to register pressure. Spilling to stack could have caused 50%+ slowdown.
486 era you needed assembler to work around quirks like AGI stalls.
On Pentium the reason for assembler was to use FPU efficiently in parallel with normal code (FPU per pixel divide for perspective correction). Of course you also needed to carefully hand optimize for Pentium U and V pipes. If you did it correctly, you could execute up to 2 instructions per clock. If not, you lose up to half of the performance (or even more if you messed up register dependency chains, which were a bit weird sometimes).
One also needs to remember compilers in the nineties were not very amazing at optimization. You could run circles around them by using assembler.
Mind you, I still need to write some things in assembler even on modern x86. But it's pretty little nowadays. SIMD stuff (SSE/AVX) you can mostly do in "almost assembler" with instruction intrinsics, but without needing to worry about instruction scheduling and so on.
I must have been a bloody hard and interesting work, back then.
Back then assembly language was much more ergonomic than it is now, and the machines were simpler. Many of my favourite games in the 80s were made by teenagers programming in their bedrooms. Check out this 68000 assembly tutorial video from Scoopex, the Amiga demo scene group: https://youtu.be/bqT1jsPyUGw
He gets a simple graphical effect going on the Amiga in only a few lines of assembly. Doing the same thing using DirectX in c++ would take you all day!
You should use SDL for this and not DirectX. You don't need complex API for GPU to paint line if you concerned about complexity.
That's just piling on even more abstraction! You may as well use a hex editor and create a single line BMP, then "run" it with any image viewer.
No need to go full shit ape and creation of 3D images in bmp is excellent exercise you should try.