Your argument is essentially (a) and (b) combined. That you don't find it valuable (however you choose to define "necessary") and you envision that the environmental impact could be lower and still provide the same function.
You say "you don't find valuable". Its not just me. People, in general, don't find cryptocurrency valuable. Nothing backs them up, and besides that you cannot widely use them in the clearnet/sneakernet.
What happened is it hasn't replaced the default currencies such as USD and EUR. Its been mainly adopted by criminals/crooks (e.g. drugs trafficking), tax evaders, people who live in banana republics (hyperinflation), speculators, and a bunch of libertarians/preppers.
So next time you say "you don't find it valuable", please realize you're talking to the vast majority of honest, casual people in the world. Of the above mentioned groups, the _only_ group I have respect for using cryptocurrency, is the people who live in banana republics (such as Venezuela).