The original ULEZ didn't raise the revenue as expected. However I'd suggest once you start to move further out the ratio of older vehicles starts to increase dramatically. These will be the working class who generally no longer live close to central London.
There is another reason, nearer the centre public transport is much more frequent and goes in multiple directions, and car clubs are more common, so some have dispensed with cars.
The further out you go, these all become less, and it is often difficult to make journeys to just outside London borough boundary (but still within M25) without a car, so having a car is more common.
The policy is not about cutting emissions (nationally), but just doing it locally. As the sold older cars just get moved to other more rural areas, rather than upgraded or scrapped.
As others have said, over time older vehicles get replaced anyway (at about average of 30000-40000 a week), regardless of any ULEZ