For nightly use in Guernsey, judge replacement by cleanliness, support and the feel you still get from the futon. Our owner's three-to-four-year reference comes from daily use and regular care in Japan. It is personal experience, not a promised lifespan for a different household.
Does a thinner futon mean it has failed?
Fibre filling settles as it receives weight over time. HIMARI's approximately 10 cm and SORA's approximately 12 cm starting lofts are nominal specifications. Consider whether the current support still suits you; a date or a single thickness measurement does not answer that on its own.
Can the older one become guest bedding?
Only consider keeping it if it remains clean, dry and suitable to sleep on. Inspect it after storage as well as after use. An older futon with mould or unresolved dampness should not simply be folded away for the next visitor.