Most (all?) items left on loungers are NOT in the valuable category, unless the book is a priceless manuscript π. So Regentβs liability may be minimal.
I agree OPβs letter may have been so arrogant as to be ignored. But a word while onboard, stressing satisfying the majority at the risk of offending the minority , might be more successful in dedicating a staff member to βpoliceβ the area.