I can't help but conclude that routine maintenance would have detected something long before the wheel came off. 7 years is far too long to have never been removed for bearing and brake inspection.
I also have to ask how you concluded that the locking washer was never set correctly at the factory? If the wheel came off the spindle, the nut, tab, washer and bearing all came off with it; making it very hard to say the washer was loose for 7 years before this happened.
2020 Eagle HT 30.5 CKTS Modern Farmhouse (16" wheels, spring shackles set on lower hole of frame, SumoSprings TSS-107-40, standard kingpin, B&W Companion slider, TST-507 TPMS)
2019 F250 SRW SuperCab 4X4 SB 6.2 (10,000# GVW) 3.73 Axle 18" wheels