Which is a bit of lazy writing to be entirely honest. "Everything has gone down the drain, no one knows how or why it works, everything is antiquated and falls apart, we're doomed...well, until that guy over there came along. Now we can do absolutely everything again.". Kind of like Movie Scientists - yeah sure, that guy surely got his PhD at the age of 13, and can learn any field in a matter of hours to save the world. Asteroid impact? Give me 5 minutes, I'll fix that. Black hole? Okay, make it 10 minutes.
MoM doesn't take place in the "modern" Cawl age, however, and it is very easy to hand wave technological upgrades to make Primaris-level armor easily integrate with bionics/cybernetics.
Because in Master of Mankind, it's revealed that bionics can't properly interface with power armor and just armor-plating your bionics restricts movement and function.
For bionics, MoM also introduced the psychological aspect they display for their foes - that guy was maimed and mutilated, and still got back up to fight on. Half his face blown off? Whatever. Limbs missing? Just tacked on new ones, they'll work. Are you sure you want to face such a foe?
As for a MotF not replacing everything with bionics, I guess it's rather down to two reasons:
1. It's part of the internal conflict - compulsively replacing everything and shutting down any illogical thinking vs. remaining human to a degree. Having a faceless emotionless "character" defeats the purpose of a unique, named character - we can use the generic ones for that.
2. GW needs a justification for each character to turn primaris, no matter how paper-thin. There's no way to improve the weak fleshy bits when those were dumped some centuries ago.
Considering that guy still only has few visible non-bionic parts left, it doesn't matter all too much in the end.Autocorrect for the win - now we have adult cannons, not just the small ones.
[...] since adult cannons, lascannons, heavy bolters, HKMs [...]
Sound positively slanneshi to me