Saving four hours a month is great, and if you have a captive audience who is being told to use the mobile app to submit expenses, that will force adoption.

Trying to get individual users to adopt a workflow is going to be much harder.

The target audience for the mobile app might not be the users themselves, but their managers.

Also, your workflow of using Slack to enter expenses, which looks awesome btw, might actually be easier for people to use than stopping midstream to use a mobile app.