Where did that 90% stat come from anyway? Visual checks are good, but do they really stop most belt issues? I've had belts snap even when they looked fine from the outside. How do you check for hidden damage like internal fraying? Seems like there might be more to preventing failures than just looking.
Actually, that 90% figure seems pretty optimistic for just a visual check. Jessica_hunt has a point about internal damage you can't see. A belt can look perfect but have hidden cracks from heat or old age that only show up under tension. You really need to pair looking with listening for squeals and feeling for slack. Relying only on monthly glances misses a lot of problems that lead to a sudden snap.
Imagine trusting a monthly look to catch most belt issues. I mean, my car's belt seemed fine until it gave out on the highway last year. Idk, maybe it's just me but that 90% number feels like a stretch. You can't spot hidden damage just by staring at it. So yeah, visual checks are good, but acting like they solve almost everything is a bit funny.