How does "sinking lid" work as a metaphor?

Solution 1:

A local New Zealand term. Imagine a tank holding liquid. The level of the liquid represents the numbers of employees of the company. The tank might have a slight leak at the bottom, so the liquid level very slowly goes down. Or some might evaporate. That represents the slow loss of employees through resignation, death, retirement, etc. Now imagine a lid floating on the liquid. That prevents you removing liquid with a ladle, cup, or siphon. The lid represents the policy of not reducing the staff by redundancy or dismissal. As the water level sinks (goes down) so does the floating lid. The gradual loss is sometimes called 'natural wastage'.

The term 'sinking lid' is used in New Zealand more generally about policies where the level of something is allowed to reduce naturally, rather than by intervention. For example, the Wellington city council has a 'sinking lid' policy regarding licencing of poker type gambling machines in pubs and clubs. No new licences (British spelling) are granted. They call these devices 'pokie machines', I see in the Stuff (NZ) web site

A related term is 'cap'. If you put a cap on something (wages, bus fares, number of employees, whatever) you prevent that level or amount from rising above a selected limit.