The longevity part is real and it is smaller than the way it gets repeated. The better argument for rotating is a different one.
Why foam recovers. Midsole foam is compressed on every stride and it re-expands afterwards, but not instantly - full recovery takes considerably longer than the few hours between an evening run and the next morning. A shoe run daily is being compressed while still partly compressed from yesterday, and that accelerates the permanent loss of cushioning.
So giving a pair a day or two between runs does extend its useful life. How much is genuinely not well established, and anyone quoting a precise percentage is making it up.
The stronger argument is variety. Two different shoes load your feet and legs slightly differently, different stack, different drop, different geometry. Running everything in one shoe means the same tissues take the same load every time, and varying it is generally thought to be a reasonable idea for anybody running regularly.
And the boring practical one: wet shoes. At forty kilometres a week you will get caught out, and a soaked pair takes a day or more to dry properly. A second pair means you are not running in wet shoes or drying them badly on a radiator, which does wreck them.