With conventional methods price and size of the spaceship portion dedicated to propulsion scale with the mass it's carrying. If we could build a massive spaceship that costs a lot to build but runs cheaply, doesn't need refueling with scarce fuels available at only few refueling facilities, carries incredible amounts of goods and is reusable for many decades or even centuries without significant service/retrofitting, it'd make sense.

Lead time is a barrier of business. If I can get my goods to any customer anywhere in the belt in less than 30 days instead of many months, it's very valuable.

You're thinking about "[no] one", I am thinking about deliveries for millions of customers per trip. We're talking about a literal warp engine, this is not near future. There will be hundreds of millions of humans in space when this engine is production ready.