The issue is that if most coins are securities, then that means in order to operate, Coinbase has to become a registered securities exchange in order for it to do what it does. The regulations for securities exchanges are onerous, but also many of them aren't possible to comply with for crypto because of how it works.

So the SEC is saying "You are an unregistered securities exchange"

And coinbase is saying "Please give some kind of route by which we could possibly be compliant given the nature of cryptocurrencies"

And the SEC is saying "We have no obligation to give you such a route, just as we have no obligation to provide such a route for criminal black markets"

Who you interpret as being unreasonable here depends on if you think crypto is closer to the black market example, or if it's closer to being like regular securities but with a few tweaks to the rules to make it work.