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.
While a good idea in theory, such currencies are never going to be very popular as people loath easily visible and (externally) enforced inflation.
It is also not really all that different from a currency issuing entity regularly making additional money and causing inflation that way. Silvio Gesell developed his theories in contrast to largely gold pegged currencies of his time, and one can argue that fiat currencies of today are already largely implementing his ideas albeit on a larger scale, making the regional benefits less apparent.
The way to make this work is imho to clearly link the inflation to funding of widely agreed on common infrastucture spending through.