I think long-term, it's completely likely that certain crypto-currencies will rise to a point of semi-stability and become legitimate "currencies," rather than merely speculative assets, because there are just so damn many people who have 0 faith current institutions. The "decentralized finance" people are behaving rationally within their questionable fundamental assumptions about the world.
The smaller currencies are absolutely comparable to MLMs, except instead of SAHMs being preyed upon, it's a bunch of dude-bros on either end of these massive pump-and-dump schemes. It's just what we saw with GME with a bunch of psychopaths exploiting all these "HODL" and "diamond hands" memes to prey upon the easily-duped. As long as the people in-the-know sell before the idiots who fall prey to the memes, they can turn a little profit on every new meme coin and keep dreaming of becoming crypto-millionaires and never working another day in their life.
But I do think that the major currencies (BTC, ETH) will continue to go up (just with occasional temporary crashes whenever they face regulatory setbacks from major countries). Eventually, the "real" currencies will stabilize as anyone who has any interest in adopting them will have finally entered the market. Then BTC and ETH (and who's to say what else might enter that tier, if anything) will function basically as gold does. Something that is functionally worthless, but contains value in a way that's decoupled from inflation or the fate of any particular nation's currency, just because a bunch of doomsday preppers all agree it's valuable, and some more fearful investors hop on board simply as a way of diversifying their portfolios. The difference is that gold doesn't consume the same amount of electricity as the entire state of Washington on an annual basis.
I do believe that in the medium term, the trustworthy/mainstream cryptocurrencies are a "good" investment if your fundamental value of what's "good" is what will appreciate in value the most. The market is not yet saturated with all the people who will eventually be interested, because like I said, trust in institutions is horrifyingly low and the "DeFi" angle of crypto has a lot of allure to large segments of the population. But the monetary gain to be had doesn't balance out the moral implications of the carbon emissions associated with crypto mining.
Arguably investing in index funds might also not be particularly "moral" in that a large proportion of the index is going to companies like Amazon and Facebook (these two are even in "socially conscious" indexes, because they aren't actively profiting off anything as egregious as oil or bombs, just consumerism and misinformation...) that have horrible effects on society and the climate, and are incredibly exploitative of their workers. So maybe it's just holier-than-thou bullshit to object to Crypto on moral grounds while investing in these corporations. There's perhaps some argument to be made that at least these companies are "adding value" and not just sitting there doing nothing, but then there's just as much of an argument that these companies are doing more harm than good, and personally profiting off their success just incentivizes rationalizing away ones' complicity in that harm.
I guess here's where I awkwardly trail off as I realize maybe my thoughts are not fully formed. Maybe if I'm fine with index investing in horrible companies that I think do great harm to society, then I should be fine with the harm associated with crypto, in which case it would be perfectly rational to buy some BTC and/or ETH alongside ones' index investing. I guess I don't really have any skin in the game yet because I need to pay off my student loans before I worry about what assets to purchase, and by the time I get there, the major crypto-currencies might be much closer to stabilizing and no longer be as attractive of investments.
But yes, the the behavior around memecoins is basically MLMs. These crypto-bros are just trying to get more people "under" them to buy in, and then they can jump out with their profits once they've conned enough people into pumping up the price.
