Well duh: you use prioritised queues precisely when there is a capacity limit. In lots of cases, the facility is specifically mandated to achieve as close to full capacity as possible.
Which is not to agree with the claim that latency queues and priorities can't achieve latency goals. Your hard requirements establish a minimum viable capacity, and you fill in the bubbles with softer work. Priorities let you distinguish between hard and soft, and to offer fairness among soft.
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Well duh: you use prioritised queues precisely when there is a capacity limit. In lots of cases, the facility is specifically mandated to achieve as close to full capacity as possible.
Which is not to agree with the claim that latency queues and priorities can't achieve latency goals. Your hard requirements establish a minimum viable capacity, and you fill in the bubbles with softer work. Priorities let you distinguish between hard and soft, and to offer fairness among soft.