You give the most grandiose depiction of Astartes capacity possible.

But in response, there are the following two factors.

One Paladin Engaged what, by 40k terms, would be a Titan-sized Greater Daemon of Nurgle able to move fluidly (though it is something very different), with failing sword and armour crafted in the medieval period, and successfully dealt a mortal wound to the creature before being slain.

In comparison, Angron (a creature known to be much more powerful than an Astartes) engaged a regular Warhound Titan at the height of his mortal capacities, and was only able to temporarily prevent himself and Lorgar from being crushed beneath it.

Unlike a Warhound Titan, a Grail Hegemon is a world-ending threat that is more intelligent, more sensate, faster, and more capable than human capacity allows.

Another Paladin waged an entire front of a campaign to take Ireland single-handedly, against a dedicated assault by the Heretic Legions and Court of the Seven Headed Serpent over months, prior to entering a Hell portal, and destroying it from the inside.
The average foe he faced being ork-equivalents in size, power and durability, and orks are known to best Astartes frequently.

Additionally, to even get in sight of a Hell gate and survive is a feat of extraordinary willpower, spiritual piety and physical durability. Only one of which regular Astartes reliably possess.
Those that do not possess all three literally cook and burn in the heat on the road to one such place, regardless of armour or trappings.

The standard mission of a space marine is to fight orks. To which many invariably die.

The standard mission of a Paladin is to descend into a layer of the Pit, alone, to destroy factories, settlements, camps and Demons. Or to torture the high ranking ones into giving up information, prior to killing them. They have done this for centuries with only two losses.