Telling an aircraft it has 15+ minutes delay still doesn't mean they can slow right down, some routes have level restrictions that must be met and can be impossible at the optimum speed so 280kts+ may be necessary.
But also many of the "level by" points are not about separation but were established years ago because crews were not managing their STAR descent profiles properly and were often unable to achieve the standard speed and min stack. On most of the sectors this is the case and if there is significant holding then it is less likely that they need to make all of the standard levels.
I find using a phrase "expect FL150 at XXX reduce to minimum speed to achieve that" usually works.