It would be more like a couple of weeks or 1 month. I see what you mean here but I think that some customers may be interested in this approach because at this point they can offer suggestions for features and influence what is included in the app.

Showing them a mockup or signing them up for email updates at least would be something tangible so they have some piece of mind between the time of purchase and launch.

Tesla did this with the Model S, no? :)

It does work in real life, if done correctly, for the right products/customers. You can give a deep discount (50% or whatever), you can also promise to return their money back if you don't deliver by a certain date etc. In way, isn't that how Kickstarter works?

This is a great way to check if there is a paying market for your idea.

Peter Levels https://levels.io/ offered early access to his currently-writing book ... and published it officially 2.5 years later. I could be wrong about the exact timeline but you get the point.