The thing about supersoldiers is, if you've got the tech to make them, you've got the tech to make them redundant. "A human but stronger and more durable" in a conflict fought with hyperadvanced biotech would be equivalent to
a cavalry charge in gasmasks during the first world war, a futile attempt at maintaining the relevance of a fundamentally obsolete military unit, only good as set dressing for dieselpunk alternate history stories not actual military effectiveness. More
practical uses of the technology would be
"create a superintelligence, a posthuman as smart compared to baseline humanity as we are compared to apes and put them in charge of your technological R&D and tactics" or
"make a super-deadly plague after vaccinating or editing the genes of your side so that they'll be asymptomatic carriers".
"Replace yourself with a more competitive species" is a questionable idea even if you had the capability.
Bioweapons are of limited military value since forever, especially since nuclear weapons exist, if your side can do that kind of gene editing and the other can't and won't even notice because they can't do Fort Detrick stuff, you probably don't need this kind of stuff anyway.
No matter how nasty diseases you can cook up, they won't do jack shit on a battlefield if enemy infantry starts wearing sealed suits. Unless you take your bioengineering to Tyranid levels, in which case that's just a solution to the drone infantry problem by biological means.
Indeed.Or swarm of small drones acting on their own to destroy enemy.
That's a better idea and probably would come alongside any supersoldiers. Probably even sooner, in reality. The issue is making them capable of doing everything infantry needs to do.
The problem with all of these solutions is, they're risky on an existential scale. Mightn't it be easier to learn to live with the enemy you've got than getting humanity obliterated by a genuinely superior genetically augmented Master Race, a superplague or in your proposal, killer robots? The very traits of "useful for fighting humans and surviving despite humanity's best efforts to the contrary" you'd want to make them militarily effective also make them dangerous if/when you lose control.
Now you are exaggerating. The classic supersoldier (tougher, faster, stronger) is only slightly more dangerous than your average former spec ops soldier, and those also tend to be physically superior to 99% of the population already, yet you don't see massive waves of daring crimes led by those. At least outside of third world and some mafias after fall of Soviet Union.
Of course treating your soldiers like dirt is never a great idea, but it becomes truly terrible when applied to the elite and very capable ones, only then the whole loyalty question becomes a major problem, though it also depends on the system of government they operate under. Even in 40k they are considered nobility and given more leeway than most other imperial people and organizations.
