We need altcoins as long as we interact economically over the net, with people we do not know (or simply with people outside our local communities). If we try to imagine what qualities a “solarpunk coin” should have, what are your thoughts? Do we know any existing coin, checking all boxes? This question is more about political economy of Solarpunk, then its technology.
As you said, money is debt. The only thing we need is for some solarpunk coins would be a platform to control that debt. Not needing to input value into the system, even just having max debt per person would be enough to have balance. The struggle would be onboarding people, as every good project that has no real lucrative side lol