If Ethereum is a utility platform and the value of its coin rises, how can it continue to be used as a utility when its used as a store of value?
Most people that buy Ethereum aren't using it, they're trading.
I actually beg to differ, people buying Ethereum are probably using it for decentralized applications that are already being run on the ETH blockchain.
Going back to your topic, Ethereum can also be used as a good store of value because all indicators point to it's value going nowhere but upwards. There is only innovation and enhancement ahead of the Ethereum blockchain and there is very little chance that some obscure little bug would be found in the code or some other catastrophe because of the sheer number of people constantly auditing the Ethereum blockchain and it's protocols.