Someone worked out that the grass verges of roads in the UK was the equivalent of Dorset. I reckon that unused lawns must cover a similar area. I can here the drone of a mower at the moment.
What does a lawn do for the environment? On the positive it probably has a carbon balance to the better. If allowed the buttercups, etc. may have a few days of providing pollen. The negative – fuel consumption. And then the third aspect – IF the lawn was something else – meadow, a couple of bushes, roses, wild flowers, we could do the maths again.
Increase in oxygen – tick. Increase in insects -tick. Increase in predators – tick. Increase in biodiversity – tick. Increase in visual stimulation – tick. Possible increase in aural impact – tick. My garden at times looks a bit of a mess compared to my neighbours. But mine can offer them 20 types of tea, the possibility of seeing butterflies and moths, seasoning for pizza and pasta. Or a green lawn with stripes! At least green woodpeckers enjoy a lawn for a brunch occasionally.