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.
Partly, I answered here: https://slrpnk.net/post/14539748/11723741
As for the scope of it, I see it as a bridge solution for the stage where we still have to interact with capitalist realm, but can already start building our own (post-capitalist, solarpunk, anarchist - whatever the name) real economy, so we need an inter-community exchange token. There should (nor will) not be a uniform solution for every community involved, so we need something to use as a mediating value carrier.
As such, it may retire at some point in favour of something better suited to the new world needs, or survive - like gold today - in mostly symbolic role.