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.
One technical remark:
Should read “deflation”.
Other than that, I agree - within your assumptions of a well run governemnt. However, there is a whole lot of people within solarpunk realm who do not believe thet “a well run government” can exist (detailed definitions aside) and do want government (governance body) beyond the scale of a community that can directly control it. Thus, any such government-backed currency would still be local. Which puts us back to the field one, lacking the intergovernmental exchange carrier.
Getting a bit more theoretical, I am in favor of two currency-backing values that are IMO elementary and not substitute: energy and time. We see many “timebank” systems run locally, but even if we can say that this is the currency created (though commitment) by individuals, they still follow the pattern of central clearing office, sometimes with added reputation systems.
Energy - which essentially all solar, by the way - is mostly used via proof-of-work schemes, and could probably be used through proof-of-useful-work filter, which to me looks like a good way to fund/support public works.
This way or another, whatever is the source of the currency, we always need to address the risk of double spending and forgery. In absence of a commonly trusted authority (bank or government), public trust and control can be exercised through an openly available, tamper-proof and distributed ledger. Now, blockchain is one way to implement it. Can we imagine something else?
tl;dr: No authority, no blockchain - then what?