William Hinman, the Director of the Division of Corporation Finance at the U.S. Securities and Exchange Commission, stated earlier this summer that "based on my understanding of the present state of ether, the Ethereum network, and its decentralized structure, current offers and sales of ether are not securities transactions".
Referencing Investopedia’s definition, a security “is a fungible, negotiable financial instrument that holds some type of monetary value. It represents an ownership position in a publicly-traded corporation (via stock), a creditor relationship with a governmental body or a corporation (represented by owning that entity’s bond), or rights to ownership as represented by an option”. If you own ethereum, bitcoin, or most other cryptocurrencies you do not own a portion of any entity. There is no ownership position created when ethereum, etc. is acquired by an individual.
Decentralization is a well known characteristic of cryptocurrencies and one of the reasons why many cryptos should not be declared as securities. Creators, developers, and people like Vitalik Buterin cannot directly influence the price of "their" cryptos. Unlike stock prices, which can be influenced by multiple factors developing from inside the company.
Decentralization exists in the crypto ecosystem thanks to the global community of nodes, consensus algorithms, and the process of mining (a popular way cryptos come into existence, e.g. bitcoin). Due to the fact there are many “miners” around the world and many “nodes” running certain blockchains on their computers globally, bitcoin, ethereum, etc. exist and operate without any central authority. Therefore, owning any amount of bitcoin or ethereum doesn’t represent having stake in an entity because there doesn't exist any entity to have a stake in. Many other cryptocurrencies operate this way, but we are seeing a rise in asset tokenization and security tokens. Both of these types of crypto assets are considered securities and would be regulated as such.
If bitcoin, ethereum, or the like are ever declared securities by the SEC, that would mean all the buying/selling of them on U.S. exchanges would be illegal unless the exchange applies for a special license. A large number of cryptocurrencies don't have any use case scenarios besides being traded for profit. If a crypto does have utility value, the probability of that value seriously being recognized are low because the blockchain ecosystem and the multiple decentralized applications are still in their early stages of development.
Fortunately, the SEC and other U.S. regulatory agencies are taking due diligence when it comes to cryptocurrency regulation. Their future official decisions regarding which cryptocurrencies are considered securities will be backed by good reasoning. Hopefully.