In-universe I guess the argument would (or should) be that they're this aging relic of the imperium's golden age kept going through sheer momentum & essentially holding the feudal logistics of the imperium hostage through ancient treaties and vows of loyalty, just enough to shore up the crumbling imperium while never actually achieving any lasting victory. Their gear slowly goes downhill as knowledge is lost, their geneseed succumbs to the various flaws/mutations, their numbers whittled down through attrition.
This justification doesn't work with the pre-Heresy era.
For that I'd just say the emperor is a fascist egomaniac who wanted an ubermensch to build his empire and brutally put down anyone who opposed him. For all the talk of the imperium being for mankind the regular rank-and-file proles are constantly murdered without a second thought for little to no reason. Summary execution is the norm, you can get turned into a servitor because you failed to meet a quota, or just shipped off to one of the forever wars where you get killed in Space Afghanistan by some dude who just doesn't want to be enslaved by this fascist theocratic state. At best the emperor views humanity as beings fit only to be enslaved and exploited until death (and beyond, hello corpse-starch) so it's unsurprising he would want a separate warrior class that are loyal only to him and his handcrafted tools/sons (oops). He didn't want to give humanity the tools of their liberation, he wanted to make tools to oppress them.
In short, the space marines fighting aliens real good is a secondary function, the primary function of space marines was having a force of unbreakably loyal psycho-indoctrinated child soldiers who will gun down striking workers and anyone else that has even the slightlest issue with the imperium.
it's unsurprising he would want a separate warrior class that are loyal only to him and his handcrafted tools/sons (oops)
This contradicts the lore. The Emperor and Malcador knew that the Heresy would happen in advance and were preparing in advance. They knew that those soldiers would not be loyal to the Emperor. Unless your point is that he wanted to deliberately arm the other side, in which case I don't see what sources you can back your claim with.
Look I'll admit I play fast and loose with 40k lore, I like 40k despite GW, not because of them. They had/have some interesting ideas for the setting but I don't take their word as gospel as quite frankly it's all made up so I happily ignore what I don't like. I don't know the deep lore of the emperor & malcador because it's more interesting to me when we barely knew anything about that entire period. I really don't like the idea that he saw it coming tbh, it's boring and verges too close to the "he's a literal god" part of 40k. So fair enough if he knew half would be traitors and not unbreakably loyal, but that's one of the parts of the lore I'll continue to not bother with.
I really don't like the idea that he saw it coming tbh
Neither do I.
My overarching point is that at least 40k is a wasted setting. It has a bunch of compelling ideas which it fails to utilise.
In general, I find it not worth in the slightest to engage with the setting. I'm an unfortunate person who hyperfocused on reading about it randomly at some point about 20 years ago.
This justification doesn't work with the pre-Heresy era.
For that I'd just say the emperor is a fascist egomaniac who wanted an ubermensch to build his empire and brutally put down anyone who opposed him. For all the talk of the imperium being for mankind the regular rank-and-file proles are constantly murdered without a second thought for little to no reason. Summary execution is the norm, you can get turned into a servitor because you failed to meet a quota, or just shipped off to one of the forever wars where you get killed in Space Afghanistan by some dude who just doesn't want to be enslaved by this fascist theocratic state. At best the emperor views humanity as beings fit only to be enslaved and exploited until death (and beyond, hello corpse-starch) so it's unsurprising he would want a separate warrior class that are loyal only to him and his handcrafted tools/sons (oops). He didn't want to give humanity the tools of their liberation, he wanted to make tools to oppress them.
In short, the space marines fighting aliens real good is a secondary function, the primary function of space marines was having a force of unbreakably loyal psycho-indoctrinated child soldiers who will gun down striking workers and anyone else that has even the slightlest issue with the imperium.
This contradicts the lore. The Emperor and Malcador knew that the Heresy would happen in advance and were preparing in advance. They knew that those soldiers would not be loyal to the Emperor. Unless your point is that he wanted to deliberately arm the other side, in which case I don't see what sources you can back your claim with.
Look I'll admit I play fast and loose with 40k lore, I like 40k despite GW, not because of them. They had/have some interesting ideas for the setting but I don't take their word as gospel as quite frankly it's all made up so I happily ignore what I don't like. I don't know the deep lore of the emperor & malcador because it's more interesting to me when we barely knew anything about that entire period. I really don't like the idea that he saw it coming tbh, it's boring and verges too close to the "he's a literal god" part of 40k. So fair enough if he knew half would be traitors and not unbreakably loyal, but that's one of the parts of the lore I'll continue to not bother with.
Neither do I.
My overarching point is that at least 40k is a wasted setting. It has a bunch of compelling ideas which it fails to utilise.
In general, I find it not worth in the slightest to engage with the setting. I'm an unfortunate person who hyperfocused on reading about it randomly at some point about 20 years ago.