Ask most procurement or operations teams which supplier worries them most, and they will usually name a recent addition, a smaller player, or one with a known history of problems. The supplier that has been reliable for eight years rarely comes up, and that is exactly the risk worth naming.
Long, trouble-free relationships accumulate a kind of institutional trust that stops getting re-examined. Contracts roll over without renegotiation. Backup options that were qualified years ago are assumed to still be viable, without anyone checking. Pricing that made sense in a different market keeps getting approved out of habit.
The risk is not that this supplier will suddenly become unreliable. It is that nobody would notice early if their situation changed, because the relationship has quietly moved from "actively managed" to "assumed fine." A change in their ownership, financial position, or capacity can go unnoticed for months precisely because the relationship stopped being checked.
A simple corrective is to schedule a genuine re-evaluation for your longest-standing suppliers on the same cycle as newer ones, not a lighter one. Confirm the backup option is still real. Ask what has changed on their side in the last two years, not just yours.
Trust is earned over time, but it should not be the reason a relationship stops being reviewed. The suppliers worth the most scrutiny are often exactly the ones that feel like they no longer need it.