Ackchyually, grass is almost always a mixture of different species (ryegrass, fescue and bluegrass) and therefore not a monoculture.

A lawn has its uses in parks, for example to have a Picknick. If you trow in some clover, daisies and ribwort, don’t spray and patches are left to grow to provide shelter it’s not that bad for biodiversity, depending on the climate of course.