I appreciate your viewpoint, your garden looks great, but that short grass is a barren wasteland for biodiversity. You have to work out what's important for you, and all sides should respect that decision.

It would be possible to maintain some of it as a native wildflower meadow instead, and keep the overall length managed without using weed killers and poisons and huge amounts of water like grass requires. There's no such thing as a native weed, but you do have to remove some individuals for diversity and soil health if they grow too large. I do this with brambles and other large light blocking species, as I don't have the space to support them, even though they're great for wildlife, they end up having a short term negative biodiversity impact in a small space. Even a small corner or container of native wildflower would support native pollinators and vastly improve the health and biodiversity of your entire garden. Get a solitary bee hotel, if they exist in your country. Leave a pile of sticks/logs somewhere for insects. Get a pond if you can. You already compost so that's good. Nature isn't meant to be tidy, neat or uniform. But I understand that not everybody can appreciate the value that could be gained from millions of gardens improving thier biodiversity, and that conforming with others and past practices and traditions may be a stronger factor for some people to want to keep their gardens neat. You're clearly a good gardener, but no wildlife conservationist, you can have both though. It's not about being intentionally messy, it's about creating conditions for wildlife to be invited in. Those fully overgrown gardens are probably great for nature, but you are right that they have to consider the size of the space and the proximity of neighbours before doing that, and not being considerate of that can make them a bit of an asshole.