I believe SEC should also focus on which of those altcoin protocols/networks are technically feasible/which of them are actually working and which of them are scams. That would probably be the SEC's best chance of shutting them down, and issue an order to have those coins delisted.
I agree partly. Of course they cannot simply "shut them down" if they weren't launched in the US - but of course if they were delisted from all US exchanges, their success chance should be reduced drastically as the US is an important market. And yes, they should concentrate on indicators which make it likely a coin is a scam.
However I think really determining "technical feasibility" here is difficult to achieve. Let's have the question for example: "Is Proof of stake a feasible algorithm?"
Proof of Stake makes some assumptions about parameters like liveness and synchronicity of nodes, but if these assumptions are true, then they seem to work (empirically - Ethereum isn't constantly attacked successfully, and even much smaller coins seem to "just work"). What every Bitcoin maximalist will tell you, however, is that these protocols work only because the liveness/synchronicity is achieved through centralization.
As both points of view could be considered valid, I guess thus it is impossible to determine in a clear way if an algorithm works or not.
Thus I think that SEC and similar authorities should simply concentrate on regulating the
communication which is required from centralized altcoin dev groups, in the same way securities issuers (e.g. public corporations) are regulated. That means for example, that they have to inform people about the main characteristics of the coin, a possible pre-mine or dev fee, the way the tokens are used (e.g. in the case of utility tokens) and of course the inherent risks - above all, that the coin may be worthless if the dev team shuts down. And also restrictions: that they can't promise any kind of benefit (as long as there is no fixed interest rate, for example).
The way the European MiCa regulation did this, seems to me a feasible way (requiring a whitepaper with these informations from all centralized altcoins, including ETH, which can also be provided by traders if the coin issuers aren't providing it).