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.
Because that’s not how human nature works. Trust starts to weaken with scale. In a small village you’d be accountable and your life would be made harder for taking advantage of someone, but if you don’t know the person and will never see her again, people feel very safe to risk and make an extra. The consequences of social life differ widely in small and big scale. Plenty of old tales of travelling merchants selling junk for gold, profitting on ignorance.
Solarpunk is optimistic. You’re probably looking for Green Growth or Cyberpunk.
You don’t need to use an altcoin, you can use the real legal tender of many countries you “don’t trust” to buy stuff. Buy Argentine currency and use it to trade. You need to trust something at some point, and what you’re (maybe unknowingly) doing is trusting authoritarian institutions. Code cannot substitute for that.
Your argument seems to be “Authoritarianism is a reasonable thing to sacrifice in order to enable trustless purchases” in the same breath as “But also I don’t even trust the authorities”, assuming you’re arguing for an altcoin.