The case has not been dropped, and I think pretty clearly states the SEC's opinion, which is quite consistent with what I said above. It is not legal to run a crypto exchange in the US (under current security laws I should add).
Under the Howey test (a security is "a contract, transaction or scheme whereby a person invests his money in a common enterprise and is led to expect profits solely from the efforts of the promoter or a third party") this is pretty clearly an unregistered security IMO. Whereas e.g. using Gemini solely to exchange N USD for M BTC with the customer retaining ownership the whole time is probably not. So had the Winklevi stuck to being a traditional exchange, they could have avoided this specific issue.
Of course, the SEC seems to believe that many shitcoins are also securities ("Buy this token for a % share in our company! Totally not equity!" is... not a convincing argument). But that's not what the complaint you linked is actually about.
Yes. You put money with them, they invest it, you get some return, maybe. That's unquestionably a security.
Note that there's real staking, as with Etherium validators, where you lock up some Etherium in exchange for a cut of the gas fees. You're still exposed to Etherium going down, but the risk to the principal is low. Then there's fake staking, where you lend someone assets which they then control and invest. That's not "staking", it's an investment deal. A bad deal, because it has a capped upside and an unlimited downside. Someone else thinks that they can get a higher return than you can, but you take the risk if they fail. See Terra/Luna, BlockFi, etc.
Notably, Bitcoin is not a security. But every stablecoin, or any other more complex system that is in some way related to something that exists in the real world is.
So you could run a Bitcoin exchange, but if you add any of the new, complex stuff you will end up getting shut down by SEC.
What makes you say that?
Any crypto that imply some kind of stake or ownership of something that exists outside of that instrument are securities. This includes all stablecoins, all defi, and probably plenty of other things I don't even know about. But pure currencies that are backed by nothing other than scarcity and other people's expectation about their value are not securities. They are either commodities or currencies. Which of the two they are is still unclear and will probably have to be litigated at some point, as this distinction has other tax and regulatory significance, but neither option gets SEC on your ass the way unregistered securities does.
This is in fact an argument against the usefulness of crypto/blockchains in general -- unlike some proponents claim, you really can't actually use tradable crypto to do anything interesting, like name resolution or whatever, because if you can freely buy and sell the tokens, as soon as you can do anything more interesting with them they probably become securities.
Not to suggest that I'm speaking in favor of them, due to bad security properties, I wouldn't... and not to say that they don't run afoul of non-SEC rules... I'm just not aware of the rationale that they're securities.