Better than an answer I'm going to give you another perspective. Testing is not your goal, it's just a tool, your specific project has risks and testing is the perfect tool to mitigate some risks.
So the question is, how risky is to build a single screen app with little UI compared to an app with a nice and friendly UI flow? When your project is the first app, then the risks after skipping end to end UI tests are mostly the same, when you build the second one, you know the risks of having problems with different web browsers, Android quirks or phone models are very real and not small, so in this case end to end UI tests do mitigate those risks very effectively.
A simple analogy, suppose you work in a technology demo without external dependencies and no plan to deploy an application. In this case, there is no shipping risk that a Continuous Integration environment could mitigate, so you can spend time setting up CI, but it's obviously not a risk.
Finally, you might not know the answer as to where is your risk, in that case, ask the business how much they want to invest in UIs now and in the future.
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Better than an answer I'm going to give you another perspective. Testing is not your goal, it's just a tool, your specific project has risks and testing is the perfect tool to mitigate some risks.
So the question is, how risky is to build a single screen app with little UI compared to an app with a nice and friendly UI flow? When your project is the first app, then the risks after skipping end to end UI tests are mostly the same, when you build the second one, you know the risks of having problems with different web browsers, Android quirks or phone models are very real and not small, so in this case end to end UI tests do mitigate those risks very effectively.
A simple analogy, suppose you work in a technology demo without external dependencies and no plan to deploy an application. In this case, there is no shipping risk that a Continuous Integration environment could mitigate, so you can spend time setting up CI, but it's obviously not a risk.
Finally, you might not know the answer as to where is your risk, in that case, ask the business how much they want to invest in UIs now and in the future.