There's nothing run-of-the-mill about getting a demanding open world game built to PS4 spec up and running on the Switch. This is a huge undertaking, by far the most ambitious conversion to the Switch to date. Yeah, building it all again with custom art would've been cool, but also crazy expensive, perhaps so much so as to not be financially viable.
Unfortunatelly, it is indeed run-of-the-mill downport, there's nothing in it that even suggests anything other than scalling back almost everything possible, just like in other previously lauded ports. As I previously said few times, fiddling with ini files alone (which I've done quite a bit) will gave you fairly good idea what you can do with TW3 to make it run on low specs.
As for art - they already have all the art, eveything is set in place in that world...all they would need to do is reshape it to something like in those reworked screenshots or to something different, maybe Borlderlands style, whichever...and in this day and age when mods with better qualitty assets than vanilla versions are popping all over the place for free and in world where you can find quite good artist for more than very reasonable prices, no, I don't think it would be very expensive.
But since they don't need to do it, as shown by success of other muddy and blurry Switch downports, it comes down to better ROI vs something that stands out...and they've made a choice...shitty from perspective of what this port could've been, but sound business choice.