If an eSIM-enabled phone has a SIM card in it (being used), what happens to the phone’s existing voice/SMS services if an eSIM data plan from a separate operator is added?

The short answer is that those services will still work, unless the user chooses to configure their phone not to. The device’s operating system shows how to select SIM profiles as primary or secondary, meaning users can predefine how to make and receive calls/SMS and consume data. They can even have two phone numbers pointing at the same phone – these settings are very easy to change if needed.