Punk music was born as an expression of the cultural and economic alienation of youth in Thatcher's UK. Cyberpunk was the extrapolation of this into a world defined by the trends of the 1980s: accelerating technology that dehumanizes, unchecked globalization, runaway capitalism that leaves a permanent underclass to make do with the castoffs of the elite. Its enduring popularity is no doubt due to the fact that these trends are very much with us today.
Offshoots adapted this to the fears of other eras: Steampunk replaced accelerating solid-state technology with industrial-revolution mechanical technology. Dieselpunk traded it for the terrifying innovations in mechanized warfare of the early 20th Century.
Egalitarian societies living in clean cities powered by sustainable energy are not punk; they are worlds that would lack the need for a punk subculture. They are hopeful, utopian visions – and just as dystopias warn, the value of utopias is to inspire.
Maybe "solar utopianism" doesn't sound as cool as "solarpunk." But calling it what it is will let us appreciate it for what it is.