Why would I assume the title is accurate? I've never heard this criticism of fiat currency before, since the whole point is that it doesn't rely on on scarcity but on the stability of the issuing body. Can you explain, or is that outside the scope of this thread?
The title isn't accurate, so I can't assume it is.
Fiat currency works precisely because the planet is finite. What you're thinking of is a currency that's tied to a finite resource, like a gold standard. Fiat currency works precisely because monetary supply is able to be increased to keep pace with both the population and economic growth. Likewise, fiat currency can be removed from the economy when the economy shrinks (although this hasn't happened before).
what is a good way for the working class (90%+ of all humans) to save
Gonna sound like a dangerous lemmy communist, but vote for parties who want to raise wages and join Union to defend your rights. The main problem is that most working class struggle to finish the month without a negative bank account, once it's solved, saving become possible again
The objection about a "finite planet" is about capitalism, not currency. A 100% communist system can still have fiat currency and function perfectly well, the two aren't even related.
Fiat can work fine on a finite planet, most money doesn't even take up physical space anymore, just bits in an account ledger database. Ideally in the future governments will make sure their currency is backed or based on labor time, and nothing is technologically preventing that, nor is crypto-currency required for it.
The working class cannot win without overthrowing the owner class. The owner class is the ruling class and functions, under capitalism, to extract from the working class and make capital into more capital. It cannot escape this role so long as capitalism exists and the owner class is in charge. The fundamental mechanisms by which that system works is coercive on both classes. "Nice" members of the owner class are hammered into complacency through failure and exit (becoming working class again) or abandonment of their principals. Or they luck out and are minor and largely irrelevant, facing no competitors or predators. Real solutions require that we organize and spread class consciousness.
On an individual level, you can try to protect yourself from some of the most extreme economic violences, but they are inherently limited. Fiat currency only has value because the issuer is "good for it" and you can use it as capital and for personal purchases of commodities. Crypto is not money at all, it is an unlicensed security. If your interest is in money-like things, I would recommend inflation hedge-alikes gold and real estate. But these require you to already have significant savings. And they are something to hawk in order to leave the country and cannot replace a functional economic base or allow you to weather a true crisis staying in the country. Having a backup shelf-stable food supply and means to boil water is also a good idea.
Our fates are all tied together under this economic system. We will quickly starve and die of preventable disease in a real, sustained crisis, as it will disrupt agriculture and utilities. Only a stable productive base, a real economy that produces what humans need, can provide when borders close or trade halts. And, realistically, everything you can do as an inflation hedge is much better when done at the community level. Mutual aid is more effective than a personal bean stash (do both!). A network of like-minded people can secure travel and estimate when to leave vs. fight. You can buy real estate with less capital if you go in together. Etc etc.
Realize that all forms of currency are ultimately a grift to allow a class of violent weirdos to claim ownership over the fruits of our labor and that we could actually just share things with each other without commerce
Buy jewelry, gold and silver coins, gemstones, rolls of silk, fine pelts, expensive spices like vanilla beans (avoid saffron though, its perishable), fine porcelains and maybe a rare painting or two. Put it all in a big wooden chest with a big iron padlock on it, and bury it in your backyard. Then draw some sort of cryptic map to it in case you die unexpectedly, or forget where your backyard is or something.
The only way for a working class person to have any financial security is to join or build a strong union with other working class people. The rich/powerful/elites will always be able to crush us if we don't work together
Disclaimer: I work in accountancy (Commercial Finance) but my advice should be taken with a pinch of salt.
Diversify your investments. Have savings across multiple bank accounts so that the unlikely event where one bank fails doesn't wipe out your financial wealth., and overall don't put your eggs in one basket.
Currently, the best ROI/passive income you'll get is from corporate bonds. A cursory glance at the Hargreaves Lansdown website shows me quite a few corporate bonds and gilts that have a voucher rate (% return each year) of over 8 percent. Do bear in mind that a bond is a loan instrument where the ROI is based on both the maturity date (the date you get paid back plus interest) and the overall risk of the debt.
Another good option is to invest in high dividend yield shares or ETFs. These can offer superior returns on investment to the interest you'd get from a savings account.
Avoid day trading or swing trading unless you 100% absolutely know what the fuck you're doing. Unlike what many YouTube 'gurus' claim, making a living off the stock market requires a high amount of starting capital to see any kind of tangible ROI, plus sophisticated stock trading software and knowledge on how to exploit trends.
Same goes for penny stocks (i.e. the FTSE AIM All Share market.) You can make some rookie gains from it. Many of the companies listed on this index are oil/gas/mineral exploratory firms that are likely not going to see any kind of profit unless they strike it big, find a new oil well, lithium mine, etc and get the necessary contracts and investment to extract those resources. They're penny stocks for a reason.
Side hustles. For example, I have two friends that are karaoke DJs and they get gigs where bars pay them to host nights. Bookkeeping is another one, but you really are going to have a lot of competition in that respect, and you will probably need the right qualifications to do it.
Screw crypto. At best cryptocurrencies are speculative assets that are scarce and expensive because the very nature of how their blockchains were designed make it take exponentially more effort to mine a new coin. At worst, they're used to launder criminal proceeds (Monero is a good example of one where the blockchain is encrypted.) NFTs are even worse.
What do you mean fiat doesn't work on a finite planet? Current economic models certainly don't work on a finite planet, but fiat was here before them and will be here after they are long gone.
what is a good way for the working class (90%+ of all humans) to save and succeed in this current environment?
There isn't one. A big chunk of that class can do just fine and you probably already have good normative answers in that respect, but the current economic model is one that demands poverty. Even with all of the ridiculous developments in production we have, the available infrastructure even with the qualms we might have with it, and all the other things going for us that you might want to list, the closest that the current economic model has achieved to escaping its age-old need for having a sizeable portion of the able-bodied population unemployed is by slightly expanding that same portion and then having them sell themselves by the hour and minute in the Gig Economy. If you want that whole 90% of the population to all be able to do well, you need to change the system they are operating within.