i don’t work directly with these guidelines, but i’m told that whoever does maintenance has to follow the maintenance intervals dictated by boeing alone.

if a plane doesn’t experience much wear, the intervals can be elongated. in addition, the maintenance company can change certain parts of the maintenance if they have the right qualifications.

but no one really checks every single nut and bolt, so delta could’ve also been sloppy.

Because otherwise airlines buy different planes. All airplane models have extremely detailed maintenance schemas with alternative procedures described where possible. And minimum equipment lists that describes exactly what must work and what is “okay” to be broken to still fly. And it’s on FAA to make sure Delta is following these manuals. So in the end the blame is on Boeing for either bad parts, lasting shorter than required or prescribing insufficient maintenance procedures. Or it’s on FAA for not doing ther duty in making sure the procedures are followed. Of course if Delta hasn’t followed the procedures, blame is on them too, but only ever in combination with either Boeing or FAA.