I’ve had entire hordes, or in maelstrom even a double boss fight during a horde, without a single marked enemy for 40+ seconds. There might have even been the odd bruiser or shooter, but then you have to rely on your teammates not accidentally offing them, and the AI not taking a break because of the amount of enemies around, leaving your marked target idle behind a wall.
I’d rework both of those keystones to have gradual falloff with different qualifiers. Granted, I will never have the opportunity to do so, but I will declare my conviction nonetheless. 
I suppose for the veteran getting a sabre we could move over to this thread.
I disagree with the notion that giving everyone every weapon is not a potential balancing issue. I know it’s not unheard of, because I’ve seen one, but I don’t know why a psyker would ever wield a mk3 knife when they have the dueling swords instead. Granted, this is alleviated somewhat by the knives having blessings that synergize with bleed - which the psyker has no source of - and both Veteran and Zealot having Talents that grant bleed stacks and/or also benefit off them.
I also like having some differentiation between classes through available weapons.
However, there is also the opportunity to differentiate the same weapons between different classes.
After all, Psykers can get force push on Las Pistols.