Is there a statute of limitations or a similar concept? For example, hypothetically would it be possible 30 years after the Ethereum ICO for the SEC to declare that ETH has been a security the entire time? As of now, it's been almost 10 years since the Ethereum ICO.

5 years unless there is evidence of active fraud.

but that doesn't mean that if something exists for more than five years it can't be a fraud. It means they would have to find an action by Coinbase that was illegal and was done within the last five years. If it was illegal for coinbase to be a broker continually up until the present day then that would be within the statute of limitations

If it was a security, but no longer is, the sale of an unregistered security back then was still a violation of the law that (barring statutes of limitations) could presumably still be prosecuted. The sale of ETH today would not be if it has transitioned into commodity status.

The Howey test would be applied to the status of the asset at the time of the alleged violation of the law that is being prosecuted.

Right, but past violations of the law have nothing to do with how an asset should currently be regulated, which is what this thread is about.

It has plenty to do with it.

People are going "well they say BTC and ETH are OK, why not this random shitcoin?"

The answer is "ETH wasn't OK when it was still like said random shitcoin".